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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a