ui: make the large file warning limit fully configurable
While add --large can be used to override it selectively, often enough
the user simply doesn't care about machines with less than 100MB RAM or
so, so make it possible to just specify a larger limit in hgrc.
Differential Revision: https://phab.mercurial-scm.org/D3893
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL