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
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors