New ‘git-fetch’ download method

[Ludovic Courtès]

Commit 9b5b5c1 adds the (guix git-download) module, which provides the
‘git-fetch’ download method.  It’s currently used by only one package,
libwebsockets:

  (package
    (name "libwebsockets")
    (version "1.2")
    (source (origin
              ;; The project does not publish tarballs, so we have to take
              ;; things from Git.
              (method git-fetch)
              (uri (git-reference
                    (url "git://git.libwebsockets.org/libwebsockets")
                    (commit (string-append "v" version
                                           "-chrome26-firefox18"))))
              (sha256
               (base32
                "1293hbz8qj4p27m1qjf8dn97r10xjyiwdpq491m87zi025s558cl"))
              (file-name (string-append name "-" version))))

    ;; ...

It’s meant to be used only in cases where upstream does not publish
tarballs, as is the case here (the project tags versions in the repo,
just doesn’t upload tarballs.)

Cheers,
Ludo’.