absorb: fix interactive mode I didn't know existed
While investigating a bug in hg absorb -e I unintentionally
discovered hg absorb --interactive and its brokenness. This adds a
test and restores the functionality.
Note that this interface is still marked experimental, so we can
change this to be more sophisticated in the future.
Differential Revision: https://phab.mercurial-scm.org/D6411