merge: pass wctx to batchremove and batchget

We would like to migrate direct calls of repo.wvfs/wwrite/wread/etc to a
call on the relevant workingfilectx, both as a cleanup (to reduce the number of
working copy functions on `repo`), and also to facilitate an in-memory merge
that doesn't write to the working copy.

In order to do that, the first step is to ensure we pass the target wctx around
and perform our writes and reads on it. Later, this object might become a
memctx.

mercurial/merge.py

@@ -1078,7 +1078,7 b' def calculateupdates(repo, wctx, mctx, a'
 
     return actions, diverge, renamedelete
 
-def batchremove(repo, actions):
+def batchremove(repo, wctx, actions):
     """apply removes to the working directory
 
     yields tuples for progress updates
@@ -1122,7 +1122,7 b' def batchremove(repo, actions):'
                            "(consider changing to repo root: %s)\n") %
                          repo.root)
 
-def batchget(repo, mctx, actions):
+def batchget(repo, mctx, wctx, actions):
     """apply gets to the working directory
 
     mctx is the context to get from
@@ -1222,14 +1222,16 b' def applyupdates(repo, actions, wctx, mc'
 
     # remove in parallel (must come first)
     z = 0
-    prog = worker.worker(repo.ui, 0.001, batchremove, (repo,), actions['r'])
+    prog = worker.worker(repo.ui, 0.001, batchremove, (repo, wctx),
+                         actions['r'])
     for i, item in prog:
         z += i
         progress(_updating, z, item=item, total=numupdates, unit=_files)
     removed = len(actions['r'])
 
     # get in parallel
-    prog = worker.worker(repo.ui, 0.001, batchget, (repo, mctx), actions['g'])
+    prog = worker.worker(repo.ui, 0.001, batchget, (repo, mctx, wctx),
+                         actions['g'])
     for i, item in prog:
         z += i
         progress(_updating, z, item=item, total=numupdates, unit=_files)