git: Require Guile-Git 0.3.0 or later.

git: Require Guile-Git 0.3.0 or later.

* guix/git.scm (auth-supported?): Remove.
(clone*): Inline code that was dependent on AUTH-SUPPORTED?.
(update-cached-checkout): Likewise.
(resolve-reference): Remove check for 'object-lookup-prefix' and use it
unconditionally.
(load-git-submodules): Remove.
(update-submodules): Use 'repository-submodules', 'submodule-lookup',
etc. unconditionally.
(update-cached-checkout): Use 'repository-close!' unconditionally.
* m4/guix.m4 (GUIX_CHECK_GUILE_GIT): New macro.
* configure.ac: Use it and error out when it fails.
* doc/guix.texi (Requirements): Bump to Guile-Git 0.3.0.