bundle: introduce per-engine compression level
If experimental.bundlecomplevel.$engine is set, prefer it over the
generic experimental.bundlecomplevel. Given that compression levels have
widely different meanings across engines, this allows much saner
configuration.
Differential Revision: https://phab.mercurial-scm.org/D3393
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2