tests/helper-runtests.sh

perf: support measuring bdiff for all changeset related data

The --all argument changes the behavior of `perfbdiff` to pull
in fulltext revision pairs for all changes related to a changeset.
The p1 and p2 manifests will be bdiffed against current. Every file
that changed between p1 and current will have its file revisions
loaded and bdiffed.

This mode of operation effectively measured the bdiff time required
for `hg commit`.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL