git-gui: Fix mouse cursor behavior when in widgets.

The mouse cursor (at least on Windows) seemed to be picking up the
cursor from the sash controls and then never resetting itself back
to the standard text cursor (the I-beam) when it was over a text area
that the user can edit (like the commit buffer) or over a text area
the user can copy from (like the diff viewer).

So now we always set the cursor to left_ptr (which according to the Tk
documentation should be available everywhere) and only for the two text
areas which we use to list file names, as the user clicks in these but
is not permitted to select text.

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

git-gui

@@ -1563,7 +1563,7 @@ proc unclick {w x y} {
 
 set font_ui {Helvetica 10}
 set font_diff {Courier 10}
-set maincursor [. cget -cursor]
+set cursor_ptr left_ptr
 
 switch -glob -- "$tcl_platform(platform),$tcl_platform(os)" {
 windows,*   {set M1B Control; set M1T Ctrl}
@@ -1690,8 +1690,8 @@ label .vpane.files.index.title -text {Modified Files} \
 text $ui_index -background white -borderwidth 0 \
 	-width 40 -height 10 \
 	-font $font_ui \
+	-cursor $cursor_ptr \
 	-yscrollcommand {.vpane.files.index.sb set} \
-	-cursor $maincursor \
 	-state disabled
 scrollbar .vpane.files.index.sb -command [list $ui_index yview]
 pack .vpane.files.index.title -side top -fill x
@@ -1707,8 +1707,8 @@ label .vpane.files.other.title -text {Untracked Files} \
 text $ui_other -background white -borderwidth 0 \
 	-width 40 -height 10 \
 	-font $font_ui \
+	-cursor $cursor_ptr \
 	-yscrollcommand {.vpane.files.other.sb set} \
-	-cursor $maincursor \
 	-state disabled
 scrollbar .vpane.files.other.sb -command [list $ui_other yview]
 pack .vpane.files.other.title -side top -fill x
@@ -1791,8 +1791,7 @@ text $ui_comm -background white -borderwidth 1 \
 	-relief sunken \
 	-width 75 -height 9 -wrap none \
 	-font $font_diff \
-	-yscrollcommand {.vpane.lower.commarea.buffer.sby set} \
-	-cursor $maincursor
+	-yscrollcommand {.vpane.lower.commarea.buffer.sby set}
 scrollbar .vpane.lower.commarea.buffer.sby \
 	-command [list $ui_comm yview]
 pack $ui_coml -side top -fill x
@@ -1866,7 +1865,6 @@ text $ui_diff -background white -borderwidth 0 \
 	-font $font_diff \
 	-xscrollcommand {.vpane.lower.diff.body.sbx set} \
 	-yscrollcommand {.vpane.lower.diff.body.sby set} \
-	-cursor $maincursor \
 	-state disabled
 scrollbar .vpane.lower.diff.body.sbx -orient horizontal \
 	-command [list $ui_diff xview]