merge: write manifestmerge() using dictionary with entry per file In the same vein as 38e55e55ae4d (largefiles: rewrite merge code using dictionary with entry per file, 2014-12-09), rewrite manifestmerge() itself as dictionary with the filename as key. This will let us simplify some of the other code in merge.py and eventually drop the conversion in the largefiles code. No difference in speed could be detected (well within the noise level when run in Mozilla repo).