The previous regex was too strict and did not match what git was allowing. It
could lead to `fetchGit` not accepting valid branch names, even though they
exist in a repository (for example, branch names containing `/`, which are
pretty standard, like `release/1.0` branches).

The new regex defines what a branch name should **NOT** contain. It takes the
definitions from `refs.c` in https://github.com/git/git and `git help
check-ref-format` pages.

This change also introduces a test for ref name validity checking, which
compares the result from Nix with the result of `git check-ref-format --branch`.

As `git fetch` may chose to interpret refspec to it's liking, ensure that we
only pass refs that begin with `refs/` as is, otherwise, prepend them with
`refs/heads`. Otherwise, branches named `heads/foo` (I know it's bad, but it's
allowed), would be fetched as `foo`, instead of `heads/foo`.

…tchgit

Improve ref validity checking in fetchgit