git-gui (Windows): use git-gui.exe in Create Desktop Shortcut

When calling `Repository>Create Desktop Shortcut`, Git GUI assumes
that it is okay to call `wish.exe` directly on Windows. However, in
Git for Windows 2.x' context, that leaves several crucial environment
variables uninitialized, resulting in a shortcut that does not work.

To fix those environment variable woes, Git for Windows comes with a
convenient `git-gui.exe`, so let's just use it when it is available.

This fixes #448

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-gui/lib/shortcut.tcl

@@ -11,11 +11,14 @@ proc do_windows_shortcut {} {
 		if {[file extension $fn] ne {.lnk}} {
 			set fn ${fn}.lnk
 		}
+		# Use git-gui.exe if available (ie: git-for-windows)
+		set cmdLine [auto_execok git-gui.exe]
+		if {$cmdLine eq {}} {
+			set cmdLine [list [info nameofexecutable] \
+							 [file normalize $::argv0]]
+		}
 		if {[catch {
-				win32_create_lnk $fn [list \
-					[info nameofexecutable] \
-					[file normalize $::argv0] \
-					] \
+				win32_create_lnk $fn $cmdLine \
 					[file normalize $_gitworktree]
 			} err]} {
 			error_popup [strcat [mc "Cannot write shortcut:"] "\n\n$err"]