memctx: calculate manifest more efficiently Before this patch, "memctx._manifest" updates all entries in the (parent) manifest. But this is inefficiency, because almost all files may be clean in that context. On the other hand, just updating entries for changed "files" specified at construction causes unexpected abortion, when there is at least one newly removed file (see issue4470 for detail). To calculate manifest more efficiently, this patch replaces "pman.iteritems()" for the loop by "self._status.modified" to avoid updating entries for clean or removed files Examination of removal is also omitted, because removed files aren't treated in this loop (= "self[f]" returns not None always).