Working fetch command.

Currently need to add the config value gerritnumber (the gerrit number
from the URL in the web page, or _number from the ChangeInfo object) to
the branch's config.

Ex: If you are on branch "my_branch" and the url to your change is
http://gerrit.dev.returnpath.net/#/c/27436/ then you need to add the
config like:

git config --add branch.my_branch.gerritnumber 27436

Change-Id: Ideefecdbee422e7a6ddf4fd883ddac2ce2e323bd