If "fetcher" is a string, then Nix will execute it with bash already, so
the additional bash argument in that string was redundant and apparently
causes trouble on non-Linux platforms.
Hopefully fixes https://github.com/NixOS/nixpkgs/issues/11496.
This is useful when `leaveDotGit = true` and some other derivation
expects some branch name to exist.
Previously, `nix-prefetch-git` always created a branch with a
hard-coded name (`fetchgit`).
This patch resolves https://github.com/NixOS/nixpkgs/issues/6395. Deep
cloning is useful in combination with 'leaveDotGit' for builds that want
to run "git describe" to obtain a proper version string, etc., like the
'haskellngPackages.cabal2nix' package does.
The name detection didn't work for e.g. http://git.suckless.org/sinit/.
I tested the tarball builds now.
@shlevy claimed nixpkgs requires nix-1.8 features anyway,
so the additional check with message were superfluous.
I think it takes the recent N commits into the repository, which says very little,
even for wanting master/HEAD.
svn path=/nixpkgs/trunk/; revision=18277
