gclient: fetch only what's needed

To reduce server load, only fetch the ref we need.

This is a redo of: http://codereview.chromium.org/2856038

First upload was the original CL 2856038. Subsequent uploads
are my changes.

TEST=ran presubmit tests

Review URL: http://codereview.chromium.org/2842060

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@53072 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -257,7 +257,13 @@ class GitWrapper(SCMWrapper):
     # Update the remotes first so we have all the refs.
     for _ in range(10):
       try:
-        remote_output, remote_err = scm.GIT.Capture(
+        if rev_type == "branch":
+          remote_output, remote_err = scm.GIT.Capture(
+            ['fetch'] + verbose + ['origin', revision],
+            self.checkout_path,
+            print_error=False)
+        else:
+          remote_output, remote_err = scm.GIT.Capture(
             ['remote'] + verbose + ['update'],
             self.checkout_path,
             print_error=False)