merge: make calculateupdates() return file->action dict This simplifies largefiles' overridecalculateupdates(), which no longer has to do the conversion it started doing in 38e55e55ae4d (largefiles: rewrite merge code using dictionary with entry per file, 2014-12-09). To keep this patch small, we'll leave the name 'actionbyfile' in overrides.py. It will be renamed in the next patch.