builtins.fetchGit: use same name as nix-prefetch-git

When using npins with (generic, i.e. non-github) git repositories, we
currently have the issue of having to download these twice: once while
doing `npins add git ...`, and then a second time when evaluating the
source attribute in Nix.

The source of this is that nix-prefetch-git, which we use to get the
hash, sets as name for the store path it produces a name derived from
the url; this behaviour is not configurable [1]. In contrast,
builtins.fetchGit sets the name as "source" by default.

The nixpkgs fetcher uses a nix implementation of the same logic to
derive the desired name in nix [2].

This commit adds a function which does the same, derived from the one in
Nixpkgs, but tweaked to only use builtin functions, and uses it to set
an appropriate name for the git fetcher.

[1] https://github.com/NixOS/nixpkgs/blob/c215fb18dc1c4ef927c773078d2365fc559365c5/pkgs/build-support/fetchgit/nix-prefetch-git#L142
[2] https://github.com/NixOS/nixpkgs/blob/2116a1123122d639b7c69e47284e4b35198c3fa6/pkgs/build-support/fetchgit/default.nix#L2

npins/default.nix

@@ -27,6 +27,7 @@ let
       revision,
       url ? null,
       hash,
+      branch ? null,
       ...
     }:
     assert repository ? type;
@@ -39,9 +40,24 @@ let
       })
     else
       assert repository.type == "Git";
+      let
+        urlToName =
+          url: rev:
+          let
+            matched = builtins.match "^.*/([^/]*)(\\.git)?$" repository.url;
+
+            short = builtins.substring 0 7 rev;
+
+            appendShort =
+              if (branch == null && (builtins.match "[a-f0-9]*" rev) != null) then "-${short}" else "";
+          in
+          "${if matched == null then "source" else builtins.head matched}${appendShort}";
+        name = urlToName repository.url revision;
+      in
       builtins.fetchGit {
         url = repository.url;
         rev = revision;
+        inherit name;
         # hash = hash;
       };