feat(third_party/git): Add derivation to build git

This overrides the upstream derivation to:

* use local sources
* build `git send-email`

It also calls autoreconf before building because files that are
included in the git distribution tarball (which the normal derivation
uses) are missing from source.

third_party/default.nix

@@ -20,6 +20,7 @@ let
     # of the repo. They become available under `pkgs.third_party.<name>`
     inherit (nixpkgs)
       age
+      autoconf
       bashInteractive
       bat
       buildGoPackage