batman: add `--no-fallback` option

If this is an existing feature, apologies! I couldn't find it. I'd go deep into my specific usecase and why this would benefit *me*, but I don't think that's necessary, as it's a fairly common-sense option.

Currently, a command like this: `batman git pasta` would fail to find a manpage for `git pasta`, and would then fallback to the manpage for `git`. However, this may not always be helpful. Sometimes, you didn't just want a general manpage, you wanted one for a specific subcommand. Rather than having to check if you're in the *right* manpage, it'd be nice to just see "No manual entry for pasta", and immediately know that the manpage doesn't exist.

Start by locating the batman command's option parsing and the fallback behavior described in the issue. Exercise `batman git pasta` with and without the proposed `--no-fallback` option; done means the option reports no manual entry for `pasta` instead of falling back to the `git` manpage.

Written by the indexing model from the issue text.