gitk: Provide a window icon if possible

Try to set up a 16x16 Tk photo image (based on the git logo) and use
it as window icon.  The code is wrapped in a catch because it may fail
in earlier Tcl/Tk 8.4 releases that don't provide 'wm iconphoto'.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -10884,6 +10884,26 @@ set lserial 0
 set isworktree [expr {[exec git rev-parse --is-inside-work-tree] == "true"}]
 setcoords
 makewindow
+catch {
+    image create photo gitlogo      -width 16 -height 16
+
+    image create photo gitlogominus -width  4 -height  2
+    gitlogominus put #C00000 -to 0 0 4 2
+    gitlogo copy gitlogominus -to  1 5
+    gitlogo copy gitlogominus -to  6 5
+    gitlogo copy gitlogominus -to 11 5
+    image delete gitlogominus
+
+    image create photo gitlogoplus  -width  4 -height  4
+    gitlogoplus  put #008000 -to 1 0 3 4
+    gitlogoplus  put #008000 -to 0 1 4 3
+    gitlogo copy gitlogoplus  -to  1 9
+    gitlogo copy gitlogoplus  -to  6 9
+    gitlogo copy gitlogoplus  -to 11 9
+    image delete gitlogoplus
+
+    wm iconphoto . -default gitlogo
+}
 # wait for the window to become visible
 tkwait visibility .
 wm title . "[file tail $argv0]: [file tail [pwd]]"