tests/helper-runtests.sh

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

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL