sync: pass --bare option when doing git clone of superproject.

Changed "git pull" to "git fetch" as we are using --bare option. Used the
following command to fetch:
  git fetch origin +refs/heads/*:refs/heads/* --prune

Pass --branch argument to Superproject's UpdateProjectsRevisionId function.

Returned False/None when directories don't exist instead of raise
GitError exception from _Fetch and _LsTree functions. The caller of Fetch
does Clone if Fetch fails.

Tested the code with the following commands.

$ ./run_tests -v

Tested the init and sync code by copying all the repo changes into my Android
AOSP checkout and running repo sync with --use-superproject option.

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707
Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I3e441ecdfc87c735f46eff0eb98efa63cc2eb22a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296222
Reviewed-by: Mike Frysinger <vapier@google.com>

git_superproject.py

@@ -29,6 +29,9 @@ from error import BUG_REPORT_URL, GitError
 from git_command import GitCommand
 import platform_utils
 
+_SUPERPROJECT_GIT_NAME = 'superproject.git'
+_SUPERPROJECT_MANIFEST_NAME = 'superproject_override.xml'
+
 
 class Superproject(object):
   """Get SHAs from superproject.
@@ -48,8 +51,9 @@ class Superproject(object):
     self._superproject_dir = superproject_dir
     self._superproject_path = os.path.join(self._repodir, superproject_dir)
     self._manifest_path = os.path.join(self._superproject_path,
-                                       'superproject_override.xml')
-    self._work_git = os.path.join(self._superproject_path, 'superproject')
+                                       _SUPERPROJECT_MANIFEST_NAME)
+    self._work_git = os.path.join(self._superproject_path,
+                                  _SUPERPROJECT_GIT_NAME)
 
   @property
   def project_shas(self):
@@ -66,8 +70,9 @@ class Superproject(object):
     Returns:
       True if 'git clone <url> <branch>' is successful, or False.
     """
-    os.mkdir(self._superproject_path)
-    cmd = ['clone', url, '--filter', 'blob:none']
+    if not os.path.exists(self._superproject_path):
+      os.mkdir(self._superproject_path)
+    cmd = ['clone', url, '--filter', 'blob:none', '--bare']
     if branch:
       cmd += ['--branch', branch]
     p = GitCommand(None,
@@ -84,15 +89,17 @@ class Superproject(object):
       return False
     return True
 
-  def _Pull(self):
-    """Do a 'git pull' to to fetch the latest content.
+  def _Fetch(self):
+    """Do a 'git fetch' to to fetch the latest content.
 
     Returns:
-      True if 'git pull <branch>' is successful, or False.
+      True if 'git fetch' is successful, or False.
     """
     if not os.path.exists(self._work_git):
-      raise GitError('git pull missing drectory: %s' % self._work_git)
-    cmd = ['pull']
+      print('git fetch missing drectory: %s' % self._work_git,
+            file=sys.stderr)
+      return False
+    cmd = ['fetch', 'origin', '+refs/heads/*:refs/heads/*', '--prune']
     p = GitCommand(None,
                    cmd,
                    cwd=self._work_git,
@@ -100,7 +107,7 @@ class Superproject(object):
                    capture_stderr=True)
     retval = p.Wait()
     if retval:
-      print('repo: error: git pull call failed with return code: %r, stderr: %r' %
+      print('repo: error: git fetch call failed with return code: %r, stderr: %r' %
             (retval, p.stderr), file=sys.stderr)
       return False
     return True
@@ -114,7 +121,9 @@ class Superproject(object):
       data: data returned from 'git ls-tree -r HEAD' instead of None.
     """
     if not os.path.exists(self._work_git):
-      raise GitError('git ls-tree. Missing drectory: %s' % self._work_git)
+      print('git ls-tree missing drectory: %s' % self._work_git,
+            file=sys.stderr)
+      return None
     data = None
     cmd = ['ls-tree', '-z', '-r', 'HEAD']
     p = GitCommand(None,
@@ -136,18 +145,19 @@ class Superproject(object):
     """Get SHAs for all projects from superproject and save them in _project_shas.
 
     Args:
-      url: superproject's url to be passed to git clone or pull.
-      branch: The branchname to be passed as argument to git clone or pull.
+      url: superproject's url to be passed to git clone or fetch.
+      branch: The branchname to be passed as argument to git clone or fetch.
 
     Returns:
       A dictionary with the projects/SHAs instead of None.
     """
     if not url:
       raise ValueError('url argument is not supplied.')
+
     do_clone = True
     if os.path.exists(self._superproject_path):
-      if not self._Pull():
-        # If pull fails due to a corrupted git directory, then do a git clone.
+      if not self._Fetch():
+        # If fetch fails due to a corrupted git directory, then do a git clone.
         platform_utils.rmtree(self._superproject_path)
       else:
         do_clone = False
@@ -208,7 +218,7 @@ class Superproject(object):
       manifest: A Manifest object that is to be written to a file.
       projects: List of projects whose revisionId needs to be updated.
       url: superproject's url to be passed to git clone or fetch.
-      branch: The branchname to be passed as argument to git clone or pull.
+      branch: The branchname to be passed as argument to git clone or fetch.
 
     Returns:
       manifest_path: Path name of the overriding manfiest file instead of None.