merge: flush any deferred writes before, and after, running any workers Since we fork to create workers, any changes they queue up will be lost after the worker terminates, so the easiest solution is to have each worker flush the writes they accumulate--we are close to the end of the merge in any case. To prevent duplicated writes, we also have the master processs flush before forking. In an in-memory merge (M2), we'll instead disable the use of workers. Differential Revision: https://phab.mercurial-scm.org/D628

filemerge: flush if using deferred writes when running a merge tool Since merge tools might read from the filesystem, we need to write out our deferred writes here. No-ops if not using deferred writes. Differential Revision: https://phab.mercurial-scm.org/D627

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