tags-cache: add a dedicated warm cache function to hgtagsfnodescache
Having a dedicated API point will help to optimize that specific usage. Right
doing a full phases weam takes a long time, even when the cache is already
filled.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL