partial-merge: add support for `.args` config (`$local` etc.) It will be useful to be able to define custom command-line arguments per partial merge tool just like we have for regular merge tools. In particular, I expect the same binary to handle multiple languages, so it will be useful to be able to pass some argument indicating the language, or perhaps simply an argument defining a regex that's used for finding lines to merge as a sorted set. Differential Revision: https://phab.mercurial-scm.org/D12383