xdiff: use int64 for hash table size
Follow-up of the previous "long" -> "int64" change. Now xdiff only uses int
for return values and small integers (ex. booleans, shifting score, bits in
hash table size, etc) so it should be able to handle large input.
Differential Revision: https://phab.mercurial-scm.org/D2765
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors