git-gui: Treat git version as git --version

We know that the version subcommand of git is special.  It does not
currently have an executable link installed into $gitexecdir and we
therefore would never match it with one of our file exists tests.
So we forward any invocations to it directly to the git wrapper, as
it is a builtin within that executable.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -294,6 +294,7 @@ proc _git_cmd {name} {
 
 	if {[catch {set v $_git_cmd_path($name)}]} {
 		switch -- $name {
+		  version   -
 		--version   -
 		--exec-path { return [list $::_git $name] }
 		}