Add rebase push display wrong comments bug (#35560) (#35580)

Backport #35560 by @lunny

Fix #35518

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/pull/pull.go

@@ -244,6 +244,17 @@ func ChangeTargetBranch(ctx context.Context, pr *issues_model.PullRequest, doer
 		}
 	}
 
+	exist, err := git_model.IsBranchExist(ctx, pr.BaseRepoID, targetBranch)
+	if err != nil {
+		return err
+	}
+	if !exist {
+		return git_model.ErrBranchNotExist{
+			RepoID:     pr.BaseRepoID,
+			BranchName: targetBranch,
+		}
+	}
+
 	// Check if branches are equal
 	branchesEqual, err := IsHeadEqualWithBranch(ctx, pr, targetBranch)
 	if err != nil {