git-gui: mc cannot be used before msgcat has been loaded

If someone attempts to use an older version that Tk 8.4 the error was
masked by the lack of a mc command.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -38,7 +38,7 @@ if {[catch {package require Tcl 8.4} err]
 	tk_messageBox \
 		-icon error \
 		-type ok \
-		-title [mc "git-gui: fatal error"] \
+		-title "git-gui: fatal error" \
 		-message $err
 	exit 1
 }