Only fetch repo once-per-day under normal 'repo sync' usage

Its unlikely that a new version of repo will be delivered in any
given day, so we now check only once every 24 hours to see if repo
has been updated.  This reduces the sync cost, as we no longer need
to contact the repo distribution servers every time we do a sync.

repo selfupdate can still be used to force a check.

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

project.py

@@ -1349,6 +1349,14 @@ class MetaProject(Project):
         if base:
           self.revision = base
 
+  @property
+  def LastFetch(self):
+    try:
+      fh = os.path.join(self.gitdir, 'FETCH_HEAD')
+      return os.path.getmtime(fh)
+    except OSError:
+      return 0
+
   @property
   def HasChanges(self):
     """Has the remote received new commits not yet checked out?