git-gui: Use arrow cursor rather than left_ptr.

Arrow is available on all Tk platforms and is mapped to the native
system cursor on Windows and Mac OS X.  Consequently its the better
cursor choice as it should match whatever the system has configured
for the standard pointing thingy.

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

git-gui

@@ -1658,7 +1658,7 @@ proc unclick {w x y} {
 ##
 ## ui init
 
-set cursor_ptr left_ptr
+set cursor_ptr arrow
 font create font_diff -family Courier -size 10
 font create font_ui
 catch {