Skip updating remotes if we already have the upstream revision.

This should make a `gclient sync` faster, especially on Windows, where git hangs regularly.

BUG=none
TEST=gclient sync -v doesn't print "Fetching origin" all the time


Review URL: https://chromiumcodereview.appspot.com/10668020

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@143898 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -353,6 +353,7 @@ class GitWrapper(SCMWrapper):
       else:
         raise gclient_utils.Error('Invalid Upstream: %s' % upstream_branch)
 
+    if not scm.GIT.IsValidRevision(cwd=self.checkout_path, rev=revision):
       # Update the remotes first so we have all the refs.
       backoff_time = 5
       for _ in range(10):