filemerge: convert a couple of wvfs calls in internal mergetools to contexts

filemerge: convert a couple of wvfs calls in internal mergetools to contexts

One hitch is that sometimes fcd is actually an absentfilectx which does not
expose any mutator functions. In order to still use the context functions,
we look up the underlying workingfilectx to perform the write there.

One alternate way would be to put the write functions on the absentfilectx and
have them pass-through. While this makes the callsites cleaner, we would need
to decide what its getter functions would return after this point, since
returning None for `data` (and True for `isabsent()`) might no longer be
correct after a write. I discussed with Sidd about just having the getters
raise RuntimeErrors after a mutator has been called, but we actually call
isabsent() in merge.py after running the internal merge tools.

--- a/mercurial/filemerge.py	Wed Jun 28 13:50:20 2017 +0200
+++ b/mercurial/filemerge.py	Mon Jun 26 22:52:15 2017 -0700
@@ -298,10 +298,10 @@
     """Uses the other `p2()` version of files as the merged version."""
     if fco.isabsent():
         # local changed, remote deleted -- 'deleted' picked
-        repo.wvfs.unlinkpath(fcd.path())
+        _underlyingfctxifabsent(fcd).remove()
         deleted = True
     else:
-        repo.wwrite(fcd.path(), fco.data(), fco.flags())
+        _underlyingfctxifabsent(fcd).write(fco.data(), fco.flags())
         deleted = False
     return 0, deleted
 
@@ -313,9 +313,19 @@
     used to resolve these conflicts."""
     # for change/delete conflicts write out the changed version, then fail
     if fcd.isabsent():
-        repo.wwrite(fcd.path(), fco.data(), fco.flags())
+        _underlyingfctxifabsent(fcd).write(fco.data(), fco.flags())
     return 1, False
 
+def _underlyingfctxifabsent(filectx):
+    """Sometimes when resolving, our fcd is actually an absentfilectx, but
+    we want to write to it (to do the resolve). This helper returns the
+    underyling workingfilectx in that case.
+    """
+    if filectx.isabsent():
+        return filectx.changectx()[filectx.path()]
+    else:
+        return filectx
+
 def _premerge(repo, fcd, fco, fca, toolconf, files, labels=None):
     tool, toolpath, binary, symlink = toolconf
     if symlink or fcd.isabsent() or fco.isabsent():