repo: Add support for standalone manifests

Added --standalone_manifest to repo tool. If set, the
manifest is downloaded directly from the appropriate source
(currently, we only support GS) and used instead of creating
a manifest git checkout. The manifests.git repo is still created to
keep track of various config but is marked as being for a standalone
manifest so that the repo tool doesn't try to run networked git
commands in it.

BUG=b:192664812
TEST=existing tests (no coverage), manual runs

Change-Id: I84378cbc7f8e515eabeccdde9665efc8cd2a9d21
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312942
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

git_config.py

@@ -104,6 +104,10 @@ class GitConfig(object):
           os.path.dirname(self.file),
           '.repo_' + os.path.basename(self.file) + '.json')
 
+  def ClearCache(self):
+    """Clear the in-memory cache of config."""
+    self._cache_dict = None
+
   def Has(self, name, include_defaults=True):
     """Return true if this configuration file has the key.
     """
@@ -399,7 +403,7 @@ class GitConfig(object):
     if p.Wait() == 0:
       return p.stdout
     else:
-      GitError('git config %s: %s' % (str(args), p.stderr))
+      raise GitError('git config %s: %s' % (str(args), p.stderr))
 
 
 class RepoConfig(GitConfig):