filemerge: in ':fail' tool, write out other side if local side is deleted

We do this because we don't want to modify the dirstate for failures, and don't
just want to leave the file missing from disk. Plus it's more useful for the
user if the changed side is written out -- it is easier to delete a file than
to get it back via hg revert.

mercurial/filemerge.py

@@ -285,6 +285,9 b' def _ifail(repo, mynode, orig, fcd, fco,'
     Rather than attempting to merge files that were modified on both
     branches, it marks them as unresolved. The resolve command must be
     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())
     return 1, False
 
 def _premerge(repo, fcd, fco, fca, toolconf, files, labels=None):