fetchFromGitiles: init

This has the same motivation as fetchFromGitHub/fetchFromGitLab --
it's cheaper to download a tarball of a single revision than it is to
download a whole history.

I could have gone with domain/group/repo, like fetchFromGitLab, but it
would have made implementation more difficult, and this syntax means
it's a drop-in replacement for fetchgit, so I decided it wasn't worth
it.