git-gui: Include browser in our usage message.

Now that the 'browser' subcommand can be used to startup the tree
browser, it should be listed as a possible subcommand option in
our usage message.

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

git-gui.sh

@@ -5478,7 +5478,7 @@ gui {
 	# fall through to setup UI for commits
 }
 default {
-	puts stderr "usage: $argv0 \[{blame|citool}\]"
+	puts stderr "usage: $argv0 \[{blame|browser|citool}\]"
 	exit 1
 }
 }