Fetch optimizations

[rtimush]

• When we know the commit hash, only fetch this commit (and its ancestors)
• When we only have a revision/branch, only fetch the relevant refs (and their ancestors).

This makes fetches significantly faster. For example, for googleapis/googleapis, it decreases the time from 1m20s to about 30s.

An even bigger improvement would be to

1. Shallow fetch. This is supported by libgit2 but I couldn't make it work.
2. Sparse checkout. This is not even supported by libgit2.

#137