mergestate: use `update_file` to handle for `ACTION_EXEC`
This is the newer, more semantic API.
Differential Revision: https://phab.mercurial-scm.org/D11169
mergestate: use `update_file` to handle for `ACTION_ADD_MODIFIED`
This is the newer, more semantic API.
Differential Revision: https://phab.mercurial-scm.org/D11168