match: use '' instead of '.' for root directory (API) I think '' is generally a better value for the root directory than '.' is. For example, os.path.join('', 'foo') => 'foo', while os.path.join('.', 'foo') => './foo'. This patch mostly makes it so we use '' internally in match.py. However, it also affects the API in visitdir(), visitchildrenset() and files(). The two former now also accept '' as input. I've updated the callers of these methods. I've also added a deprecation warning for passing '.' (for external callers). The only caller I could find that was affected by files() returning '' instead of '.' was in dirstate.walk(). I've updated that. The next few patches show some workarounds we can remove by using '' instead of '.'. Differential Revision: https://phab.mercurial-scm.org/D6401