git-gui: Bind Cmd-, to Preferences on Mac OS X

A Mac OS X UI convention is to have Cmd-, be the accelerator key
for the preferences window, which by convention is located in the
apple menu under a separator below the about command.  We also now
call this "Preferences..." as that is the conventional term used
in English.

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

git-gui.sh

@@ -2018,8 +2018,12 @@ if {[is_MacOSX]} {
 
 	.mbar.apple add command -label [mc "About %s" [appname]] \
 		-command do_about
-	.mbar.apple add command -label [mc "Options..."] \
-		-command do_options
+	.mbar.apple add separator
+	.mbar.apple add command \
+		-label [mc "Preferences..."] \
+		-command do_options \
+		-accelerator $M1T-,
+	bind . <$M1B-,> do_options
 } else {
 	# -- Edit Menu
 	#