Restore deleted branches when syncing (#29898)

Regression of #29493. If a branch has been deleted, repushing it won't
restore it.

Lunny may have noticed that, but I didn't delve into the comment then
overlooked it:
https://github.com/go-gitea/gitea/pull/29493#discussion_r1509046867

The additional comments added are to explain the issue I found during
testing, which are unrelated to the fixes.

services/repository/branch.go

@@ -318,11 +318,11 @@ func SyncBranchesToDB(ctx context.Context, repoID, pusherID int64, branchNames,
 		for i, branchName := range branchNames {
 			commitID := commitIDs[i]
 			branch, exist := branchMap[branchName]
-			if exist && branch.CommitID == commitID {
+			if exist && branch.CommitID == commitID && !branch.IsDeleted {
 				continue
 			}
 
-			commit, err := getCommit(branchName)
+			commit, err := getCommit(commitID)
 			if err != nil {
 				return fmt.Errorf("get commit of %s failed: %v", branchName, err)
 			}