maps: add the "revhashes" method to RevMap This is a part of the bigger plan to get rid of reading or writing rev_map directly without going through the RevMap class. The "revhashes" method will be used in revset_svnrev.

include Makefile hgsubversion/help/*.rst tests/fixtures/invalid_utf8.tar.gz
recursive-include tests *.py *.sh *.svndump *.txt