gclient: fetch only what's needed

gclient_scm.py

Index: gclient_scm.py
diff --git a/gclient_scm.py b/gclient_scm.py
index a76d2d0c2c882887a1ae32680b4969532a4bb2fb..928ffdf1c7b736403757ae8f5694ead825bbab1e 100644
--- a/gclient_scm.py
+++ b/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)