gitk: Use an external icon file on Windows

Git for Windows now ships with the new Git icon from git-scm.com. Use that
icon file if it exists instead of the old procedurally drawn one.

This patch was sent upstream but so far no decision on its inclusion was
made, so commit it to our fork.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>

gitk-git/gitk

@@ -12436,7 +12436,6 @@ if { [info exists ::env(GITK_MSGSDIR)] } {
     set gitk_prefix [file dirname [file dirname [file normalize $argv0]]]
     set gitk_libdir [file join $gitk_prefix share gitk lib]
     set gitk_msgsdir [file join $gitk_libdir msgs]
-    unset gitk_prefix
 }
 
 ## Internationalization (i18n) through msgcat and gettext. See
@@ -12799,28 +12798,32 @@ if {[expr {[exec git rev-parse --is-inside-work-tree] == "true"}]} {
 set worktree [gitworktree]
 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
-
-    image create photo gitlogo32    -width 32 -height 32
-    gitlogo32 copy gitlogo -zoom 2 2
-
-    wm iconphoto . -default gitlogo gitlogo32
+if {$::tcl_platform(platform) eq {windows} && [file exists $gitk_prefix/etc/git.ico]} {
+    wm iconbitmap . -default $gitk_prefix/etc/git.ico
+} else {
+    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
+
+        image create photo gitlogo32    -width 32 -height 32
+        gitlogo32 copy gitlogo -zoom 2 2
+
+        wm iconphoto . -default gitlogo gitlogo32
+    }
 }
 # wait for the window to become visible
 tkwait visibility .