maintner/maintnerd, cmd/gerritbot, cmd/gitmirror: add protobuf

Set up mirroring for the new protobuf repository:

	Gerrit:      https://go.googlesource.com/protobuf
	GitHub:      https://github.com/protocolbuffers/protobuf-go
	Import path: google.golang.org/protobuf

Change-Id: I29bb081e223fde798b439c966a3a6fb389a01a06
Reviewed-on: https://go-review.googlesource.com/c/build/+/171035
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/cmd/gerritbot/gerritbot.go b/cmd/gerritbot/gerritbot.go
index 596de0d..d0ead74 100644
--- a/cmd/gerritbot/gerritbot.go
+++ b/cmd/gerritbot/gerritbot.go
@@ -209,6 +209,7 @@
 	"perf":           true,
 	"playground":     true,
 	"proposal":       true,
+	"protobuf":       true,
 	"review":         true,
 	"scratch":        true,
 	"sublime-build":  true,
diff --git a/cmd/gitmirror/gitmirror.go b/cmd/gitmirror/gitmirror.go
index d0c72e2..2061138 100644
--- a/cmd/gitmirror/gitmirror.go
+++ b/cmd/gitmirror/gitmirror.go
@@ -231,10 +231,13 @@
 				continue
 			}
 			path := "golang.org/x/" + name
-			if name == "dl" {
+			switch name {
+			case "dl":
 				// This subrepo is different from others in that
 				// it doesn't use the /x/ path element.
 				path = "golang.org/" + name
+			case "protobuf":
+				path = "google.golang.org/" + name
 			}
 			go startRepo(name, path, false)
 		}
@@ -293,6 +296,7 @@
 		"oauth2",
 		"playground",
 		"proposal",
+		"protobuf",
 		"review",
 		"scratch",
 		"sync",
diff --git a/maintner/maintnerd/maintnerd.go b/maintner/maintnerd/maintnerd.go
index 0ed8e2f..9355604 100644
--- a/maintner/maintnerd/maintnerd.go
+++ b/maintner/maintnerd/maintnerd.go
@@ -337,6 +337,7 @@
 	"golang/vgo",
 	"golang/website",
 	"golang/xerrors",
+	"protocolbuffers/protobuf-go",
 }
 
 func setGoConfig() {