Implement git ref reading purely in Python

Its much faster to read the refs from 114 projects when the reader
is pure Python and just doing file IO than forking 114 git commands
and parsing their output.

The reader caches refs based upon file mtimes.  If any single ref
file has been modified since the last read, we re-read the entire
repository's ref namespace.  This simplifies the code as we don't
need to worry about shooting down symbolic-refs, but it may cause
more IO than is necessary if only one ref gets updated.

This change drops `repo branches` in Android from 1.658s to 0.206s.
Likewise, `repo sync` improves dramatically as well.

Signed-off-by: Shawn O. Pearce <sop@google.com>

project.py

@@ -28,11 +28,7 @@ from error import GitError, ImportError, UploadError
 from error import ManifestInvalidRevisionError
 from remote import Remote
 
-HEAD    = 'HEAD'
-R_HEADS = 'refs/heads/'
-R_TAGS  = 'refs/tags/'
-R_PUB   = 'refs/published/'
-R_M     = 'refs/remotes/m/'
+from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
 
 def _error(fmt, *args):
   msg = fmt % args
@@ -226,6 +222,7 @@ class Project(object):
     else:
       self.work_git = None
     self.bare_git = self._GitGetByExec(self, bare=True)
+    self.bare_ref = GitRefs(gitdir)
 
   @property
   def Exists(self):
@@ -301,7 +298,7 @@ class Project(object):
     """Get all existing local branches.
     """
     current = self.CurrentBranch
-    all = self.bare_git.ListRefs()
+    all = self._allrefs
     heads = {}
     pubd = {}
 
@@ -1030,32 +1027,13 @@ class Project(object):
 
   @property
   def _allrefs(self):
-    return self.bare_git.ListRefs()
+    return self.bare_ref.all
 
   class _GitGetByExec(object):
     def __init__(self, project, bare):
       self._project = project
       self._bare = bare
 
-    def ListRefs(self, *args):
-      cmdv = ['for-each-ref', '--format=%(objectname) %(refname)']
-      cmdv.extend(args)
-      p = GitCommand(self._project,
-                     cmdv,
-                     bare = self._bare,
-                     capture_stdout = True,
-                     capture_stderr = True)
-      r = {}
-      for line in p.process.stdout:
-        id, name = line[:-1].split(' ', 2)
-        r[name] = id
-      if p.Wait() != 0:
-        raise GitError('%s for-each-ref %s: %s' % (
-                       self._project.name,
-                       str(args),
-                       p.stderr))
-      return r
-
     def LsOthers(self):
       p = GitCommand(self._project,
                      ['ls-files',