rebase: sort roots by revision
Previously we were sorting them by whatever implicit sort order we got
between changectx instances. Sorting by rev at least makes some sense,
so I chose that.
Differential Revision: https://phab.mercurial-scm.org/D2301
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2