cmd/gopherbot: don't consider reopened issues in close-cherry-pick task

gopherbot isn't supposed to fight humans. If someone has re-opened
a cherry-pick issue, don't try to close it again.

Rename openCherryPickIssues to cherryPickIssues, to better reflect
that it's the set of candidate cherry-pick issues to close, not all
open ones.

Fixes golang/go#29270

Change-Id: I2b0ae9bd9849e72f7f04b208919735451c1f4593
Reviewed-on: https://go-review.googlesource.com/c/154337
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/gopherbot/gopherbot.go

@@ -1360,13 +1360,13 @@ func (b *gopherbot) getMinorMilestoneForMajor(ctx context.Context, majorRel stri
 // closeCherryPickIssues closes cherry-pick issues when CLs are merged to
 // release branches, as GitHub only does that on merge to master.
 func (b *gopherbot) closeCherryPickIssues(ctx context.Context) error {
-	openCherryPickIssues := make(map[int32]*maintner.GitHubIssue) // by GitHub Issue Number
+	cherryPickIssues := make(map[int32]*maintner.GitHubIssue) // by GitHub Issue Number
 	b.gorepo.ForeachIssue(func(gi *maintner.GitHubIssue) error {
-		if gi.Closed || gi.PullRequest || gi.NotExist || gi.Milestone.IsNone() {
+		if gi.Closed || gi.PullRequest || gi.NotExist || gi.Milestone.IsNone() || gi.HasEvent("reopened") {
 			return nil
 		}
 		if strings.Count(gi.Milestone.Title, ".") == 2 { // minor release
-			openCherryPickIssues[gi.Number] = gi
+			cherryPickIssues[gi.Number] = gi
 		}
 		return nil
 	})
@@ -1388,7 +1388,7 @@ func (b *gopherbot) closeCherryPickIssues(ctx context.Context) error {
 				if id := ref.Repo.ID(); id.Owner != "golang" || id.Repo != "go" {
 					continue
 				}
-				gi, ok := openCherryPickIssues[ref.Number]
+				gi, ok := cherryPickIssues[ref.Number]
 				if !ok {
 					continue
 				}