memctx: calculate manifest correctly with newly-removed files (issue4470) Before this patch, "memctx._manifest" tries to get (and use normally) filectx also for newly-removed files, even though "memctx.filectx()" returns None for such files. To calculate manifest correctly even with newly-removed files, this patch does: - replace "man.iteritems()" for the loop by "self._status.modified" to avoid accessing itself to newly removed files this also reduces loop cost for large manifest. - remove files in "self._status.removed" from the manifest In this patch, amending is confirmed twice to examine both (1) newly removed files and (2) ones already removed in amended revision.