Currently we specify versions for Gitlab-Shell, Workhorse and Gitaly
using version strings, to which we prepend 'v' and assume are tags.
These changes allow branches or tags with other name formats to be
specified by prepending '=' to the version string (á la govendor).

We also simplify the process to reset to the given version (now a
branch or tag): Right now there's a check to supposedly try to avoid
fetching from the remote the version if it already exist locally. But
the previous logic already clones if the directory doesn't exist or
fetches if it does, so this check is pointless. We can safely assume the
version exists once we get to the reset stage.