filemerge: add support for the new "mergediff" marker style to premerge
This adds support for the new "mergediff" conflict marker style to
"merge-tools.<tool>.premerge" (the new value is called
"keep-mergediff").
Differential Revision: https://phab.mercurial-scm.org/D9606
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..