rebase: stop relying on having two parents to resume rebase
I'm about to make it so we don't have two parents when a rebase is
interrupted (unless we're just rebasing on a merge commit). The code
for detecting if we're resuming a rebase relied on having two parents,
so this patch rewrites that to instead set a boolean when we resume.
Note that self.resume in the new condition implies `not
self.inmemory` (rebase cannot be resumed in memory), so that's why
that part can be omitted.
Differential Revision: https://phab.mercurial-scm.org/D7826