cmd/gopherbot: avoid auto-assignment of CLs on shared branches

Don't auto-assign reviewers for CLs on shared branches such as
"dev.garbage" or "dev.ssa" -- the presumption is that folks working on
such branches will be familiar enough with development practices that
they can assign their own reviewers.

Change-Id: I196d73d76cb0c6e72dd30f52f9341febff311b7a
Reviewed-on: https://go-review.googlesource.com/c/build/+/196141
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/gopherbot/gopherbot.go b/cmd/gopherbot/gopherbot.go
index 3d05e3d..3033646 100644
--- a/cmd/gopherbot/gopherbot.go
+++ b/cmd/gopherbot/gopherbot.go
@@ -1723,6 +1723,14 @@
 			if cl.Private || cl.WorkInProgress() || time.Since(cl.Created) < 10*time.Minute {
 				return nil
 			}
+
+			// Don't auto-assign reviewers to CLs on shared branches;
+			// the presumption is that developers there will know which
+			// reviewers to assign.
+			if strings.HasPrefix(cl.Branch(), "dev.") {
+				return nil
+			}
+
 			tags := cl.Meta.Hashtags()
 			if tags.Contains(tagNoOwners) {
 				return nil