gitk: Fix bug where current row number display stops working

The display of the current row number would stop working if the user
clicked on a line, or if selectedline got unset for any other reason,
because the trace on it got lost when it was unselected.  This fixes
it by changing the places that unset selectedline to set it to the
empty string instead, and the places that tested for it being set or
unset to compare it with the empty string.  Thus it never gets unset
now.  This actually simplified the code in a few places since it can
be compared for equality with a row number now without first testing
if it is set.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -495,7 +495,7 @@ proc reloadcommits {} {
 	stop_rev_list $curview
     }
     resetvarcs $curview
-    catch {unset selectedline}
+    set selectedline {}
     catch {unset currentid}
     catch {unset thickerline}
     catch {unset treediffs}
@@ -927,7 +927,7 @@ proc removefakerow {id} {
     modify_arc $v $a $i
     if {[info exist currentid] && $id eq $currentid} {
 	unset currentid
-	unset selectedline
+	set selectedline {}
     }
     if {[info exists targetid] && $targetid eq $id} {
 	set targetid $p
@@ -1838,7 +1838,7 @@ proc makewindow {} {
     pack .tf.bar.rowlabel .tf.bar.rownum .tf.bar.rowlabel2 .tf.bar.numcommits \
 	-side left
     global selectedline
-    trace add variable selectedline {write unset} selectedline_change
+    trace add variable selectedline write selectedline_change
 
     # Status label and progress bar
     set statusw .tf.bar.status
@@ -2185,7 +2185,7 @@ proc windows_mousewheel_redirector {W X Y D} {
 proc selectedline_change {n1 n2 op} {
     global selectedline rownumsel
 
-    if {$op eq "unset"} {
+    if {$selectedline eq {}} {
 	set rownumsel {}
     } else {
 	set rownumsel [expr {$selectedline + 1}]
@@ -3274,7 +3274,7 @@ proc showview {n} {
     set ytop [expr {[lindex $span 0] * $ymax}]
     set ybot [expr {[lindex $span 1] * $ymax}]
     set yscreen [expr {($ybot - $ytop) / 2}]
-    if {[info exists selectedline]} {
+    if {$selectedline ne {}} {
 	set selid $currentid
 	set y [yc $selectedline]
 	if {$ytop < $y && $y < $ybot} {
@@ -3388,7 +3388,7 @@ proc bolden {row font} {
 
     lappend boldrows $row
     $canv itemconf $linehtag($row) -font $font
-    if {[info exists selectedline] && $row == $selectedline} {
+    if {$row == $selectedline} {
 	$canv delete secsel
 	set t [eval $canv create rect [$canv bbox $linehtag($row)] \
 		   -outline {{}} -tags secsel \
@@ -3402,7 +3402,7 @@ proc bolden_name {row font} {
 
     lappend boldnamerows $row
     $canv2 itemconf $linentag($row) -font $font
-    if {[info exists selectedline] && $row == $selectedline} {
+    if {$row == $selectedline} {
 	$canv2 delete secsel
 	set t [eval $canv2 create rect [$canv2 bbox $linentag($row)] \
 		   -outline {{}} -tags secsel \
@@ -3831,7 +3831,7 @@ proc askrelhighlight {row id} {
     global descendent highlight_related iddrawn rhighlights
     global selectedline ancestor
 
-    if {![info exists selectedline]} return
+    if {$selectedline eq {}} return
     set isbold 0
     if {$highlight_related eq [mc "Descendant"] ||
 	$highlight_related eq [mc "Not descendant"]} {
@@ -4005,7 +4005,7 @@ proc visiblerows {} {
 
 proc layoutmore {} {
     global commitidx viewcomplete curview
-    global numcommits pending_select selectedline curview
+    global numcommits pending_select curview
     global lastscrollset lastscrollrows commitinterest
 
     if {$lastscrollrows < 100 || $viewcomplete($curview) ||
@@ -4916,7 +4916,7 @@ proc drawcmittext {id row col} {
 			    -text $name -font $nfont -tags text]
     set linedtag($row) [$canv3 create text 3 $y -anchor w -fill $fgcolor \
 			    -text $date -font mainfont -tags text]
-    if {[info exists selectedline] && $selectedline == $row} {
+    if {$selectedline == $row} {
 	make_secsel $row
     }
     set xr [expr {$xt + [font measure $font $headline]}]
@@ -5107,7 +5107,7 @@ proc drawvisible {} {
     if {$endrow >= $vrowmod($curview)} {
 	update_arcrows $curview
     }
-    if {[info exists selectedline] &&
+    if {$selectedline ne {} &&
 	$row <= $selectedline && $selectedline <= $endrow} {
 	set targetrow $selectedline
     } elseif {[info exists targetid]} {
@@ -5427,7 +5427,7 @@ proc dofind {{dirn 1} {wrap 1}} {
     }
     focus .
     if {$findstring eq {} || $numcommits == 0} return
-    if {![info exists selectedline]} {
+    if {$selectedline eq {}} {
 	set findstartline [lindex [visiblerows] [expr {$dirn < 0}]]
     } else {
 	set findstartline $selectedline
@@ -5623,7 +5623,7 @@ proc markmatches {canv l str tag matches font row} {
 		   [expr {$x0+$xlen+2}] $y1 \
 		   -outline {} -tags [list match$l matches] -fill yellow]
 	$canv lower $t
-	if {[info exists selectedline] && $row == $selectedline} {
+	if {$row == $selectedline} {
 	    $canv raise $t secsel
 	}
     }
@@ -5782,7 +5782,7 @@ proc appendrefs {pos ids var} {
 proc dispneartags {delay} {
     global selectedline currentid showneartags tagphase
 
-    if {![info exists selectedline] || !$showneartags} return
+    if {$selectedline eq {} || !$showneartags} return
     after cancel dispnexttag
     if {$delay} {
 	after 200 dispnexttag
@@ -5796,7 +5796,7 @@ proc dispneartags {delay} {
 proc dispnexttag {} {
     global selectedline currentid showneartags tagphase ctext
 
-    if {![info exists selectedline] || !$showneartags} return
+    if {$selectedline eq {} || !$showneartags} return
     switch -- $tagphase {
 	0 {
 	    set dtags [desctags $currentid]
@@ -6018,7 +6018,7 @@ proc sellastline {} {
 proc selnextline {dir} {
     global selectedline
     focus .
-    if {![info exists selectedline]} return
+    if {$selectedline eq {}} return
     set l [expr {$selectedline + $dir}]
     unmarkmatches
     selectline $l 1
@@ -6033,7 +6033,7 @@ proc selnextpage {dir} {
     }
     allcanvs yview scroll [expr {$dir * $lpp}] units
     drawvisible
-    if {![info exists selectedline]} return
+    if {$selectedline eq {}} return
     set l [expr {$selectedline + $dir * $lpp}]
     if {$l < 0} {
 	set l 0
@@ -6047,7 +6047,7 @@ proc selnextpage {dir} {
 proc unselectline {} {
     global selectedline currentid
 
-    catch {unset selectedline}
+    set selectedline {}
     catch {unset currentid}
     allcanvs delete secsel
     rhighlight_none
@@ -6056,7 +6056,7 @@ proc unselectline {} {
 proc reselectline {} {
     global selectedline
 
-    if {[info exists selectedline]} {
+    if {$selectedline ne {}} {
 	selectline $selectedline 0
     }
 }
@@ -6868,7 +6868,7 @@ proc redisplay {} {
     setcanvscroll
     allcanvs yview moveto [lindex $span 0]
     drawvisible
-    if {[info exists selectedline]} {
+    if {$selectedline ne {}} {
 	selectline $selectedline 0
 	allcanvs yview moveto [lindex $span 0]
     }
@@ -7189,8 +7189,7 @@ proc rowmenu {x y id} {
 
     stopfinding
     set rowmenuid $id
-    if {![info exists selectedline]
-	|| [rowofcommit $id] eq $selectedline} {
+    if {$selectedline eq {} || [rowofcommit $id] eq $selectedline} {
 	set state disabled
     } else {
 	set state normal
@@ -7214,7 +7213,7 @@ proc rowmenu {x y id} {
 proc diffvssel {dirn} {
     global rowmenuid selectedline
 
-    if {![info exists selectedline]} return
+    if {$selectedline eq {}} return
     if {$dirn} {
 	set oldid [commitonrow $selectedline]
 	set newid $rowmenuid
@@ -9890,6 +9889,7 @@ set viewperm(0) 0
 set viewargs(0) {}
 set viewargscmd(0) {}
 
+set selectedline {}
 set numcommits 0
 set loginstance 0
 set cmdlineok 0