merge: pass wctx to premerge, filemerge
In the in-memory merge branch. we'll need to call a function (``flushall``) on
the wctx inside of _xmerge.
This prepares the way so it can be done without hacks like ``fcd.ctx()``.
Differential Revision: https://phab.mercurial-scm.org/D449
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present