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
tests: work around libressl being different about error strings (issue6122)
As far as I can tell, this is the right behavior. Thanks to Alex
Gaynor for checking what the string means by looking at libressl
sources for me.
Differential Revision: https://phab.mercurial-scm.org/D6410