git-gui (Win): make starting via "Git GUI Here" on .git/ possible

This works around git-gui's error message

    Cannot use funny .git directory: .

when started from the .git/ directory, which is useful in repositories
without any directories for the right click.

Now git-gui can be started via Windows Explorer shell extension (Git GUI
Here) from the .git/ directory.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

windows/git-gui.sh

@@ -3,7 +3,12 @@
 exec wish "$0" -- "$@"
 
 if { $argc >=2 && [lindex $argv 0] == "--working-dir" } {
-	cd [lindex $argv 1]
+	set workdir [lindex $argv 1]
+	cd $workdir
+	if {[lindex [file split $workdir] end] eq {.git}} {
+		# Workaround for Explorer right click "Git GUI Here" on .git/
+		cd ..
+	}
 	set argv [lrange $argv 2 end]
 	incr argc -2
 }