The fetch command is simply an alias for:
gclient recurse -s git git fetch origin

Rework 'recurse' command to use a WorkQueue so that --jobs is supported.

BUG=115840
TEST=

Review URL: http://codereview.chromium.org/9463044

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@124334 0039d316-1c4b-4281-b951-d872f2087c98