If the working copy isn't on a named branch, don't try to
pull; just fetch.

Use --quiet flag to git-submodule, to prevent interleaving of
terminal output.

Check target os before updating a submodule.

TBR'ing, since no one really has a stake in this, and maruel doesn't
do bash :).

TBR=maruel@chromium.org
Review URL: https://codereview.chromium.org/11645056

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@174280 0039d316-1c4b-4281-b951-d872f2087c98