* guix/git-download.scm (git-package): New procedure.
(git-fetch): Use it. Remove 'git-for-build'.
Use a gexp and 'gexp->derivation'.
* guix/download.scm (gnutls-package): Fix docstring.
* guix/git-download.scm (<git-reference>)[recursive?]: New field.
(git-fetch): Add 'inputs' variable. Add it to the #:inputs argument
of 'build-expression->derivation'. Augment builder with call to
'set-path-environment-variable', and pass #:recursive? to
'git-fetch'.
* guix/build/git.scm (git-fetch): Add #:recursive? parameter. Pass
--recursive when RECURSIVE? is true, and delete all the '.git' files.