This allows all supported fetchers to be used, e.g.

  builtins.fetchTree {
    type = "github";
    owner = "NixOS";
    repo = "nix";
    rev = "d4df99a3349cf2228a8ee78dea320afef86eb3ba";
  }

Example usage:

  $ nix eval-hydra-jobs -f '<nixpkgs/pkgs/top-level/release.nix>' '' \
      --max-memory-size 2048 --workers 8

Fixes NixOS/hydra#715.

Fixes #3361.

Fixes #3363.

This reverts commit 5921ca8.

Looks like this got accidentally revived after 204291f.

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

builtins.fetchGit: Fix build when fetching a git worktree

On second thought, let's move this back to Hydra.

Hacky...

Got this mixed up somewhere.

…efinition

'nix run' will try to run $out/bin/<name>, where <name> is the
derivation name (excluding the version). This often works well:

  $ nix run nixpkgs#hello
  Hello, world!

  $ nix run nix -- --version
  nix (Nix) 2.4pre20200626_adf2fbb

  $ nix run patchelf -- --version
  patchelf 0.11.20200623.e61654b

  $ nix run nixpkgs#firefox -- --version
  Mozilla Firefox 77.0.1

  $ nix run nixpkgs#gimp -- --version
  GNU Image Manipulation Program version 2.10.14

though not always:

  $ nix run nixpkgs#git
  error: unable to execute '/nix/store/kp7wp760l4gryq9s36x481b2x4rfklcy-git-2.25.4/bin/git-minimal': No such file or directory

Fixes #3756.

This allows you to refer to an input from another flake. For example,

  $ nix run --inputs-from /path/to/hydra nixpkgs#hello

runs 'hello' from the 'nixpkgs' inputs of the 'hydra' flake.

Fixes #3769.

…lable

Until now, the `gitlab`-fetcher determined the source's rev by checking
the latest commit of the given `ref` using the
`/repository/branches`-API.

This breaks however when trying to fetch a gitlab-repo by its tag:

```
$ nix repl
nix-repl> builtins.fetchTree gitlab:Ma27/nvim.nix/0.2.0
error: --- Error ------------------------------------------------------------------------------------- nix
unable to download 'https://gitlab.com/api/v4/projects/Ma27%2Fnvim.nix/repository/branches/0.2.0': HTTP error 404 ('')
```

When using the `/commits?ref_name`-endpoint[1] you can pass any kind of
valid ref to the `gitlab`-fetcher.

Please note that this fetches the only first 20 commits on a ref,
unfortunately there's currently no endpoint which only retrieves the
latest commit of any kind of `ref`.

[1] https://docs.gitlab.com/ee/api/commits.html#list-repository-commits

Fixes #3794.

Fix gitlab-fetcher to obtain tags and branches

That is, the commands 'nix path-info nixpkgs#hello' and 'nix path-info
/nix/store/00ls0qi49qkqpqblmvz5s1ajl3gc63lr-hello-2.10.drv' now do the
same thing (i.e. build the derivation and operate on the output store
path, rather than the .drv path).

For instance, 'nix why-depends --use-derivation nixpkgs#hello
nixpkgs#glibc' shows why hello's .drv depends on glibc's .drv.