cmd/gitmirror: also server master status at /debug/watcher/

Change-Id: I3778619d9c9562b34f63e436f7e785c67ae175b0
Reviewed-on: https://go-review.googlesource.com/44034
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/gitmirror/gitmirror.go b/cmd/gitmirror/gitmirror.go
index 4ead939..393f9fe 100644
--- a/cmd/gitmirror/gitmirror.go
+++ b/cmd/gitmirror/gitmirror.go
@@ -231,8 +231,10 @@
 )
 
 // GET /
+// or:
+// GET /debug/watcher/
 func handleRoot(w http.ResponseWriter, r *http.Request) {
-	if r.URL.Path != "/" {
+	if r.URL.Path != "/" && r.URL.Path != "/debug/watcher/" {
 		http.NotFound(w, r)
 		return
 	}