git-gui: Protect against bad translation strings

If a translation string uses a format character we don't have an
argument for then it may throw an error when we attempt to format
the translation.  In this case switch back to the default format
that comes with the program (aka the English translation).
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -88,13 +88,20 @@ if {![catch {set _verbose $env(GITGUI_VERBOSE)}]} {
 
 package require msgcat
 
-proc mc {fmt args} {
-	set fmt [::msgcat::mc $fmt]
+proc _mc_trim {fmt} {
 	set cmk [string first @@ $fmt]
 	if {$cmk > 0} {
-		set fmt [string range $fmt 0 [expr {$cmk - 1}]]
+		return [string range $fmt 0 [expr {$cmk - 1}]]
 	}
-	return [eval [list format $fmt] $args]
+	return $fmt
+}
+
+proc mc {en_fmt args} {
+	set fmt [_mc_trim [::msgcat::mc $en_fmt]]
+	if {[catch {set msg [eval [list format $fmt] $args]} err]} {
+		set msg [eval [list format [_mc_trim $en_fmt]] $args]
+	}
+	return $msg
 }
 
 proc strcat {args} {