sync: Implement fetchcmd for standard Git layouts

Introduce support for `repo.fetchcmd` configuration, allowing users to
specify a custom command to fetch project data instead of
`_RemoteFetch`.

When `repo.fetchcmd` is specified, `repo` will execute it instead of
`_RemoteFetch` during the network half of sync. Note that
`repo.fetchcmd` requires `repo.uselocalgitdirs` to be enabled.

The command is executed in a subshell with project-context environment
variables, including the new `REPO_TREV` (target revision resolved to a
commit hash) and `REPO_PROJECT_FETCH_URL`.

After execution, `repo` verifies that the target commit is available and
that the tracking ref and FETCH_HEAD are correctly updated.

Tested with:
```
> ~/git-repo/repo init -u https://android-review.googlesource.com/platform/manifest \
    --repo-url file:///usr/local/google/home/gavinmak/git-repo \
    --groups developers \
    --no-repo-verify \
    --use-local-gitdirs
...

> git config --file .repo/manifests.git/config repo.fetchcmd 'mkdir -p $REPO_PATH && cd $REPO_PATH && if [ ! -d .git ]; then git init && git remote add aosp $REPO_PROJECT_FETCH_URL; fi && git fetch aosp $REPO_TREV && git reset --hard $REPO_TREV && mkdir -p .git/refs/remotes/aosp && echo $REPO_TREV > .git/refs/remotes/aosp/main && echo $REPO_TREV > .git/FETCH_HEAD'

> ~/git-repo/repo sync -j32
warning: repo is not tracking a remote branch, so it will not receive updates; run `repo init --repo-rev=stable` to fix.
You are currently enrolled in Git submodules experiment (go/android-submodules-quickstart).  Use --no-use-superproject to override.

Syncing: 100% (4/4), done in 1m15.686s
Finalizing sync state...
repo sync has finished successfully.
```

Bug: 513329573
Change-Id: I754d3f3c78e86fdeee1a72115297a75b571bc497
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/583883
Reviewed-by: Becky Siegel <beckysiegel@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

tests/test_project.py

@@ -1286,3 +1286,71 @@ class GetEnvVarsTests(unittest.TestCase):
             env = proj.GetEnvVars()
 
             self.assertEqual(env["REPO_LREV"], "")
+
+
+class FetchCmdTests(unittest.TestCase):
+    """Tests for fetch_cmd feature."""
+
+    def setUpManifest(self, tempdir):
+        repodir = os.path.join(tempdir, ".repo")
+        manifest_dir = os.path.join(repodir, "manifests")
+        manifest_file = os.path.join(repodir, manifest_xml.MANIFEST_FILE_NAME)
+        os.mkdir(repodir)
+        os.mkdir(manifest_dir)
+        manifest = manifest_xml.XmlManifest(repodir, manifest_file)
+
+        return project.ManifestProject(
+            manifest, "test/manifest", os.path.join(tempdir, ".git"), tempdir
+        )
+
+    def _get_project(self, tempdir):
+        proj = _create_mock_project(
+            tempdir, use_local_gitdirs=True, fetch_cmd="echo hi"
+        )
+        proj.GetRevisionId = mock.MagicMock(return_value="1234abcd")
+        return proj
+
+    def test_fetch_cmd_execution(self):
+        """Test that fetch_cmd is executed with correct environment."""
+        with utils_for_test.TempGitTree() as tempdir:
+            proj = self._get_project(tempdir)
+
+            proj.bare_git.rev_parse.return_value = "1234abcd"
+            mock_remote = mock.MagicMock()
+            mock_remote.ToLocal.return_value = "refs/remotes/origin/main"
+            proj.GetRemote = mock.MagicMock(return_value=mock_remote)
+
+            with mock.patch("subprocess.run") as mock_run:
+                mock_run.return_value = mock.MagicMock(returncode=0, stderr="")
+                res = proj._CustomFetch()
+
+                self.assertTrue(res)
+                mock_run.assert_called_once()
+            args, kwargs = mock_run.call_args
+            self.assertEqual(args[0], "echo hi")
+            self.assertEqual(kwargs["shell"], True)
+            self.assertEqual(kwargs["cwd"], tempdir)
+            self.assertEqual(kwargs["env"]["REPO_TREV"], "1234abcd")
+            self.assertEqual(
+                kwargs["env"]["REPO_PROJECT_FETCH_URL"],
+                "http://example.com/repo",
+            )
+
+    def test_sync_fetch_cmd_requires_use_local_gitdirs(self):
+        """Test that fetch_cmd requires use_local_gitdirs."""
+        with utils_for_test.TempGitTree() as tempdir:
+            fakeproj = self.setUpManifest(tempdir)
+
+            class DummyManifest:
+                is_submanifest = False
+
+                def GetDefaultGroupsStr(self, with_platform=False):
+                    return ""
+
+            fakeproj.manifest = DummyManifest()
+
+            fakeproj.config.SetString("repo.fetchcmd", "echo hi")
+            fakeproj.config.SetBoolean("repo.uselocalgitdirs", False)
+
+            result = fakeproj.Sync(use_local_gitdirs=False)
+            self.assertFalse(result)