git-gui: Tighten internal pattern match for lib/ directory

Our GITGUI_LIBDIR macro was testing only for @@ at the start of
the path, assuming nobody would ever find that to be a reasonable
prefix for a directory to install our library into.  That is most
likely a valid assumption, but its even more unlikely they would
have the start be @@GITGUI_ and the end be @@.  Note that we
cannot use the full string here because that would get expanded
by the sed replacement in our Makefile.

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

git-gui.sh

@@ -25,7 +25,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA}
 ## configure our library
 
 set oguilib {@@GITGUI_LIBDIR@@}
-if {[string match @@* $oguilib]} {
+if {[string match @@GITGUI_*@@ $oguilib]} {
 	set oguilib [file join [file dirname [file normalize $argv0]] lib]
 }
 set idx [file join $oguilib tclIndex]