Since fb38459d6e, each `ref` is appended
with `refs/heads` unless it starts with `refs/` already. This regressed
two use-cases that worked fine before:
* Specifying a commit hash as `ref`: now, if `ref` looks like a commit
hash it will be directly passed to `git fetch`.
* Specifying a tag without `refs/tags` as prefix: now, the fetcher prepends
`refs/*` to a ref that doesn't start with `refs/` and doesn't look
like a commit hash. That way, both a branch and a tag specified in
`ref` can be fetched.
The order of preference in git is
* file in `refs/` (e.g. `HEAD`)
* file in `refs/tags/`
* file in `refs/heads` (i.e. a branch)
After fetching `refs/*`, ref is resolved the same way as git does.
Change-Id: Idd49b97cbdc8c6fdc8faa5a48bef3dec25e4ccc3
