Split part of got_worktree_merge_prepare into a new function, got_worktree_merge_write_refs, since that part doesn't make sense in the fast-forward case. ok stsp@

Previously, this command would trigger an error: got: /: bad path

fixes "proc (null)" appearing in gotd logs

in 1a0c81fb61 I've added a range check too strict for max_repos_display, as zero is allowed and means "show all the repositories".

ok op@

This adds the functions got_repo_cleanup_prepare() and _complete() to lock/unlock the repository to prevent multiple `gotadmin cleanup' or `git gc' operations to run in parallel. improvements and ok stsp@

it's redundant with GOT_ERR_BAD_QUERYSTRING; convert its only use to the latter.

All the special cases are now being explained last.