If you do a fetchTree on a Git repository, whether the result contains
a revCount attribute should not depend on whether that repository
happens to be a shallow clone or not. That would complicate caching a
lot and would be semantically messy. So applying fetchTree/fetchGit to
a shallow repository is now an error unless you pass the attribute
'shallow = true'. If 'shallow = true', we don't return revCount, even
if the repository is not actually shallow.
Note that Nix itself is not doing shallow clones at the moment. But it
could do so as an optimisation if the user specifies 'shallow = true'.
Issue #2988.
Worktrees[1] are a feature of git which allow you to check out a ref in
a different directory.
While playing around with flakes I realized that git repositories in a
worktree checkout break when trying to build a flake:
```
$ git worktree add ../nixpkgs-flakes nixpkgs-flakes
$ cd ../nixpkgs-flakes
$ nix build .#hello
error: opening directory '/home/ma27/Projects/nixpkgs-flakes/.git/refs/heads': Not a directory
```
This issue has been fixed by determining with `git rev-parse --git-common-dir`
where the actual `.git` directory is.
Please note that this issue only exists on the `flakes` branch, fetching
worktree checkouts with Nix master seems to work fine.
[1] https://git-scm.com/docs/git-worktree
