Add doctor command for full GC of LFS (#21978)

The recent PR adding orphaned checks to the LFS storage is not
sufficient to completely GC LFS, as it is possible for LFSMetaObjects to
remain associated with repos but still need to be garbage collected.

Imagine a situation where a branch is uploaded containing LFS files but
that branch is later completely deleted. The LFSMetaObjects will remain
associated with the Repository but the Repository will no longer contain
any pointers to the object.

This PR adds a second doctor command to perform a full GC.

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/cron/tasks_basic.go

@@ -63,7 +63,7 @@ func registerRepoHealthCheck() {
 		for _, arg := range rhcConfig.Args {
 			args = append(args, git.CmdArg(arg))
 		}
-		return repo_service.GitFsck(ctx, rhcConfig.Timeout, args)
+		return repo_service.GitFsckRepos(ctx, rhcConfig.Timeout, args)
 	})
 }