Appending the branch to the fetch spec causes sync of a mirror to
fail for projects that don't have an explicit revision specified,
and don't have a branch of the same name as the default revision.
For example, a manifest defining a default revision:
<default revision="master">
having a project without an explicit revision:
<project name="path/to/project">
and not having a branch named "master", will cause repo sync to
fail for that project with the error:
Couldn't find remote ref refs/heads/master
Modify the logic to not append the branch onto the fetch spec when
syncing to a mirror.
Change-Id: I5c4457bd125519abf27abe682dea62ad708978c9
403b64edf4