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
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors