Don't list as merged branches not at mergecommitid

Once a branch has been merged if the commit ID no longer equals that of
the merge commit id don't offer to delete the branch on the pull screen
and don't list it as merged on branches.

Fix go-gitea#9201

Author: zeripath

routers/repo/issue.go

@@ -29,6 +29,7 @@ import (
 	comment_service "code.gitea.io/gitea/services/comments"
 	issue_service "code.gitea.io/gitea/services/issue"
 	pull_service "code.gitea.io/gitea/services/pull"
+	"gopkg.in/src-d/go-git.v4/plumbing"
 
 	"github.com/unknwon/com"
 )
@@ -966,6 +967,24 @@ func ViewIssue(ctx *context.Context) {
 			ctx.Data["IsBlockedByRejection"] = pull.ProtectedBranch.MergeBlockedByRejectedReview(pull)
 			ctx.Data["GrantedApprovals"] = cnt
 		}
+		if pull.HasMerged && pull.HeadRepo != nil {
+			// && git.IsBranchExist(pull.HeadRepo.RepoPath(), pull.HeadBranch) {
+			headRepo, err := git.OpenRepository(pull.HeadRepo.RepoPath())
+			if err != nil {
+				ctx.ServerError("OpenRepository", err)
+				return
+			}
+			defer headRepo.Close()
+			commit, err := headRepo.GetBranchCommitID(pull.HeadBranch)
+			if err != nil && err != plumbing.ErrReferenceNotFound {
+				ctx.ServerError("GetBranchCommitID", err)
+				return
+			} else if err == nil && commit != pull.MergedCommitID {
+				// the head has moved on from the merge - we shouldn't delete
+				canDelete = false
+			}
+
+		}
 		ctx.Data["IsPullBranchDeletable"] = canDelete && pull.HeadRepo != nil && git.IsBranchExist(pull.HeadRepo.RepoPath(), pull.HeadBranch)
 
 		ctx.Data["PullReviewers"], err = models.GetReviewersByIssueID(issue.ID)

templates/repo/branch/list.tmpl

@@ -84,6 +84,12 @@
 												<button id="new-pull-request" class="ui compact basic button">{{$.i18n.Tr "repo.pulls.compare_changes"}}</button>
 											</a>
 											{{end}}
+										{{else if and .LatestPullRequest.HasMerged (ne .LatestPullRequest.MergedCommitID .Commit.ID.String)}}
+											{{if and (not .IsDeleted) $.AllowsPulls (gt .CommitsAhead 0)}}
+											<a href="{{$.RepoLink}}/compare/{{$.DefaultBranch | EscapePound}}...{{if ne $.Repository.Owner.Name $.Owner.Name}}{{$.Owner.Name}}:{{end}}{{.Name | EscapePound}}">
+												<button id="new-pull-request" class="ui compact basic button">{{$.i18n.Tr "repo.pulls.compare_changes"}}</button>
+											</a>
+											{{end}}
 										{{else}}
 											<a href="{{$.RepoLink}}/pulls/{{.LatestPullRequest.Issue.Index}}">#{{.LatestPullRequest.Issue.Index}}</a>
 											{{if .LatestPullRequest.HasMerged}}