Removed leaveDotGit from fetchgit, since it's broken by design.

fetchgit with leaveDotGit can never be deterministic, since the
resulting .git can change whenever the upstream repository
changes.

Since it is impossible for git to download a single commit, fetchgit
downloads the whole repository and searches for the specific
commit. This means that the resulting .git contains the whole
repository, including any commit that was made after the one we
want. This also mean that adding any commit to the repository will add
it to prefetch's .git, thus changing its hash, and making it impossible
to make it deterministic.