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.

m4/guix.m4

@@ -204,6 +204,28 @@ AC_DEFUN([GUIX_CHECK_GUILE_GCRYPT], [
      fi])
 ])
 
+dnl GUIX_CHECK_GUILE_GIT
+dnl
+dnl Check whether a recent-enough Guile-Git is available.
+AC_DEFUN([GUIX_CHECK_GUILE_GIT], [
+  dnl Check whether we're using Guile-Git 0.3.0 or later.  0.3.0
+  dnl introduced SSH authentication support and more.
+  AC_CACHE_CHECK([whether Guile-Git is available and recent enough],
+    [guix_cv_have_recent_guile_git],
+    [GUILE_CHECK([retval],
+      [(use-modules (git) (git auth) (git submodule))
+       (let ((auth (%make-auth-ssh-agent)))
+         repository-close!
+	 object-lookup-prefix
+         (make-clone-options
+          #:fetch-options (make-fetch-options auth)))])
+     if test "$retval" = 0; then
+       guix_cv_have_recent_guile_git="yes"
+     else
+       guix_cv_have_recent_guile_git="no"
+     fi])
+])
+
 dnl GUIX_TEST_ROOT_DIRECTORY
 AC_DEFUN([GUIX_TEST_ROOT_DIRECTORY], [
   AC_CACHE_CHECK([for unit test root directory],