svnmeta: add the "revmapexists" method 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 revset functions "fromsvn" and "svnrev" will read the rev_map file directly, and throw an exception if the file does not exist. This makes the check possible via svnmeta.

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