Use libgit2 with ssh-exec support

See https://github.com/libgit2/libgit2/pull/6617. This ensures that we
get support for ~/.ssh/config, known_hosts etc.

src/libfetchers/git-utils.cc

@@ -336,9 +336,6 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         const std::string & url,
         const std::string & refspec) override
     {
-        /* FIXME: use libgit2. Unfortunately, it doesn't support
-           ssh_config at the moment. */
-        #if 0
         Remote remote;
 
         if (git_remote_create_anonymous(Setter(remote), *this, url.c_str()))
@@ -352,20 +349,6 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
         if (git_remote_fetch(remote.get(), &refspecs2, nullptr, nullptr))
             throw Error("fetching '%s' from '%s': %s", refspec, url, git_error_last()->message);
-        #endif
-
-        // FIXME: git stderr messes up our progress indicator, so
-        // we're using --quiet for now. Should process its stderr.
-        runProgram("git", true,
-            { "-C", path.abs(),
-              "--bare",
-              "fetch",
-              "--quiet",
-              "--force",
-              "--",
-              url,
-              refspec
-            }, {}, true);
     }
 
     void verifyCommit(