Improve sync fork behavior (#33319)

Fix #33271

Suppose there is a `branch-a` in fork repo:

1. if `branch-a` exists in base repo: try to sync `base:branch-a` to `fork:branch-a`
2. if `branch-a` doesn't exist in base repo: try to sync `base:main` to `fork:branch-a`

models/git/branch.go

@@ -167,6 +167,9 @@ func GetBranch(ctx context.Context, repoID int64, branchName string) (*Branch, e
 			BranchName: branchName,
 		}
 	}
+	// FIXME: this design is not right: it doesn't check `branch.IsDeleted`, it doesn't make sense to make callers to check IsDeleted again and again.
+	// It causes inconsistency with `GetBranches` and `git.GetBranch`, and will lead to strange bugs
+	// In the future, there should be 2 functions: `GetBranchExisting` and `GetBranchWithDeleted`
 	return &branch, nil
 }