git-gui: Refactored diff line display formatting logic.

The tags used for diff formatting (which I inherited from gitool) just
didn't make a whole lot of sense, especially if you wanted to try to
match them to the diff output you were seeing on screen.  It did not
help that the diff-index -c output's first two columns are also munged
to make the diff output more user friendly.

So this is a large refactoring of the tags used for diff display.  Now
our tag names match what we put in the left column of each line, which
makes it easier to correlate presentation and implementation.

I removed bold font usage from everything except the hunk headers as I
really did not like the way bold font caused column alignments to become
out of whack within the diff viewer.  It also drew attention to the parts
of the file which were identically changed in both the index and in the
working directory, yet these are usually the parts I find myself caring
the least about.  So its very counter-intuitive.

Lines which are changed differently by both the index and the working
directory are now shown with background colors which span the entire line,
making these lines easier to pick out of the diff.  In general these are
the lines that appear to be more interesting to me when looking at the
3-way diff as they are the ones which contain recent and quite different
changes.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui

@@ -544,7 +544,7 @@ files list, to prevent possible confusion.
 
 proc show_diff {path {w {}} {lno {}}} {
 	global file_states file_lists
-	global diff_3way diff_active repo_config
+	global is_3way_diff diff_active repo_config
 	global ui_diff current_diff ui_status_value
 
 	if {$diff_active || ![lock_index read]} return
@@ -565,7 +565,7 @@ proc show_diff {path {w {}} {lno {}}} {
 
 	set s $file_states($path)
 	set m [lindex $s 0]
-	set diff_3way 0
+	set is_3way_diff 0
 	set diff_active 1
 	set current_diff $path
 	set ui_status_value "Loading diff of [escape_path $path]..."
@@ -620,10 +620,13 @@ proc show_diff {path {w {}} {lno {}}} {
 }
 
 proc read_diff {fd} {
-	global ui_diff ui_status_value diff_3way diff_active
+	global ui_diff ui_status_value is_3way_diff diff_active
 	global repo_config
 
+	$ui_diff conf -state normal
 	while {[gets $fd line] >= 0} {
+		# -- Cleanup uninteresting diff header lines.
+		#
 		if {[string match {diff --git *}      $line]} continue
 		if {[string match {diff --combined *} $line]} continue
 		if {[string match {--- *}             $line]} continue
@@ -631,40 +634,38 @@ proc read_diff {fd} {
 		if {$line eq {deleted file mode 120000}} {
 			set line "deleted symlink"
 		}
-		if {[string match index* $line]} {
-			if {[string first , $line] >= 0} {
-				set diff_3way 1
-			}
-		}
 
-		$ui_diff conf -state normal
-		if {!$diff_3way} {
-			set x [string index $line 0]
-			switch -- $x {
-			"@" {set tags da}
-			"+" {set tags dp}
-			"-" {set tags dm}
+		# -- Automatically detect if this is a 3 way diff.
+		#
+		if {[string match {@@@ *} $line]} {set is_3way_diff 1}
+
+		# -- Reformat a 3 way diff, 'cause its too weird.
+		#
+		if {$is_3way_diff} {
+			set op [string range $line 0 1]
+			switch -- $op {
+			{@@} {set tags d_@}
+			{++} {set tags d_+ ; set op { +}}
+			{--} {set tags d_- ; set op { -}}
+			{ +} {set tags d_++; set op {++}}
+			{ -} {set tags d_--; set op {--}}
+			{+ } {set tags d_-+; set op {-+}}
+			{- } {set tags d_+-; set op {+-}}
 			default {set tags {}}
 			}
+			set line [string replace $line 0 1 $op]
 		} else {
-			set x [string range $line 0 1]
-			switch -- $x {
-			default {set tags {}}
-			"@@" {set tags da}
-			"++" {set tags dp; set x " +"}
-			" +" {set tags {di bold}; set x "++"}
-			"+ " {set tags dni; set x "-+"}
-			"--" {set tags dm; set x " -"}
-			" -" {set tags {dm bold}; set x "--"}
-			"- " {set tags di; set x "+-"}
+			switch -- [string index $line 0] {
+			@ {set tags d_@}
+			+ {set tags d_+}
+			- {set tags d_-}
 			default {set tags {}}
 			}
-			set line [string replace $line 0 1 $x]
 		}
 		$ui_diff insert end $line $tags
-		$ui_diff insert end "\n"
-		$ui_diff conf -state disabled
+		$ui_diff insert end "\n" $tags
 	}
+	$ui_diff conf -state disabled
 
 	if {[eof $fd]} {
 		close $fd
@@ -2987,12 +2988,17 @@ pack $ui_diff -side left -fill both -expand 1
 pack .vpane.lower.diff.header -side top -fill x
 pack .vpane.lower.diff.body -side bottom -fill both -expand 1
 
-$ui_diff tag conf dm -foreground red
-$ui_diff tag conf dp -foreground blue
-$ui_diff tag conf di -foreground {#00a000}
-$ui_diff tag conf dni -foreground {#a000a0}
-$ui_diff tag conf da -font font_diffbold
-$ui_diff tag conf bold -font font_diffbold
+$ui_diff tag conf d_@ -font font_diffbold
+$ui_diff tag conf d_+  -foreground blue
+$ui_diff tag conf d_-  -foreground red
+$ui_diff tag conf d_++ -foreground {#00a000}
+$ui_diff tag conf d_-- -foreground {#a000a0}
+$ui_diff tag conf d_+- \
+	-foreground red \
+	-background {light goldenrod yellow}
+$ui_diff tag conf d_-+ \
+	-foreground blue \
+	-background azure2
 
 # -- Diff Body Context Menu
 #