largefiles: refactor lfutil.findfiles to be more logical

forget: fix subrepo recursion for explicit path handling When support for handling explicit paths in subrepos was added to the forget command (95174c381525), subrepo recursion wasn't taken into account. This change fixes that by pulling the majority of the logic of commands.forget into cmdutil.forget, which can then be called from both there and subrepo.forget.

add: fix subrepo recursion for explicit path handling When support for handling explicit paths in subrepos was added to the add command (9e99d2bbb1b1), subrepo recursion wasn't taken into account. This change adds an explicitonly argument to cmdutil.add to allow controlling which levels of recursion should include only explicit paths versus all matched paths.