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
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1