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.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.