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>

git-gui/lib/shortcut.tcl

@@ -11,11 +11,17 @@ proc do_windows_shortcut {} {
 		if {[file extension $fn] ne {.lnk}} {
 			set fn ${fn}.lnk
 		}
+		# Use /cmd/git-gui.exe if available
+		set normalized [file normalize $::argv0]
+		regsub "/mingw../libexec/git-core/git-gui$" \
+			$normalized "/cmd/git-gui.exe" cmdLine
+		if {$cmdLine != $normalized && [file exists $cmdLine]} {
+			set cmdLine [list [file nativename $cmdLine]]
+		} else {
+			set cmdLine [list [info nameofexecutable] $normalized]
+		}
 		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"]