tests: test divergence created during interrupted rebase
If a rebase runs into conflicts and the user somehow rewrites an
unrebased commit in the rebase set while the rebase is interrupted,
continuing it might result in divergence. It turns out that we decide
to skip the commit. That seems to make sense, but it wasn't obvious to
me that that's what we should do. Either way, this patch adds a test
case for the current behavior.
Differential Revision: https://phab.mercurial-scm.org/D10256