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
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t