git-gui: Quote git path when starting another gui in a submodule

In do_git_gui the path of the git executable has to be put into a
list, otherwise calling it will fail when when spaces are present
in its path.

Reported-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2059,7 +2059,7 @@ proc do_git_gui {} {
 	# -- Always start git gui through whatever we were loaded with.  This
 	#    lets us bypass using shell process on Windows systems.
 	#
-	set exe [_which git]
+	set exe [list [_which git]]
 	if {$exe eq {}} {
 		error_popup [mc "Couldn't find git gui in PATH"]
 	} else {