tests/helper-runtests.sh

perf: use storage API for resolving manifest node

lookup() isn't part of the storage API. And this code shouldn't
be accessing manifestlog._revlog directly for the modern code base.
So let's port it to the modern API.

Note that the previous code was busted for cases where we needed
to call lookup() because lookup() isn't exposed by manifestrevlog
any more.

This change is strictly BC breaking because we no longer support
resolving partial nodes. But it is a perf* command and I don't
think we should flag the change as such.

Differential Revision: https://phab.mercurial-scm.org/D4390

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL