git-gui: Don't crash when starting gitk from a browser session

If the user has started git-gui from the command line as a browser
we offer the gitk menu options but we didn't create the main status
bar widget in the "." toplevel.  Trying to access it while starting
gitk just results in Tcl errors.

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

git-gui.sh

@@ -1125,11 +1125,17 @@ proc mapdesc {state path} {
 }
 
 proc ui_status {msg} {
-	$::main_status show $msg
+	global main_status
+	if {[info exists main_status]} {
+		$main_status show $msg
+	}
 }
 
 proc ui_ready {{test {}}} {
-	$::main_status show {Ready.} $test
+	global main_status
+	if {[info exists main_status]} {
+		$main_status show [mc "Ready."] $test
+	}
 }
 
 proc escape_path {path} {