fix(repo): mark repository as non-empty before git push returns (#40102)

The post-receive hook syncs pushed branches to the database, but
`is_empty` and the default branch of an empty repository were only
updated later by the `push_update` queue. Until then the API treated the
repository as empty (branch list `null`, `GET /branches/{branch}` 500),
and the queue overwrote the default branch chosen by the hook or set via
the API.

`SyncBranchesToDB` now initializes an empty repository and repairs a
default branch missing in git in its locked transaction, so the state is
final when `git push` returns. The push queue and web file edits retry
it while the repository is still empty.

- The default branch is the configured one if it exists, else `main`,
else `master`, else the first branch by name, like GitLab. HEAD follows
it.
- A first push of several branches no longer depends on push order. The
push that initializes a repository still prints no pull request hints,
like GitHub.

---------

Co-authored-by: silverwind <me@silverwind.io>

tests/integration/git_push_test.go

@@ -103,7 +103,13 @@ func testGitPush(t *testing.T, u *url.URL) {
 				doGitCreateBranch(gitPath, branchName)(t)
 			}
 			pushed = append(pushed, "master")
-			doGitPushTestRepository(gitPath, "origin", "--all")(t)
+			_, stderr, err := gitcmd.NewCommand("push", "origin", "--all").WithDir(gitPath).RunStdString(t.Context())
+			require.NoError(t, err)
+			assert.NotContains(t, stderr, "Create a new pull request")
+
+			repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerID: 2, LowerName: "repo-to-push"})
+			assert.False(t, repo.IsEmpty)
+			assert.Equal(t, "master", repo.DefaultBranch)
 			return pushed, deleted
 		})
 	})
@@ -139,7 +145,9 @@ func testGitPush(t *testing.T, u *url.URL) {
 			pushed = append(pushed, "master")
 
 			// push all, so that master is not changed
-			doGitPushTestRepository(gitPath, "origin", "--all")(t)
+			_, stderr, err := gitcmd.NewCommand("push", "origin", "--all").WithDir(gitPath).RunStdString(t.Context())
+			require.NoError(t, err)
+			assert.Contains(t, stderr, "Create a new pull request for 'branch-2'")
 
 			return pushed, deleted
 		})