git-debrebase: keycommits: Callbacks get separate $mainwhy

git-debrebase: keycommits: Callbacks get separate $mainwhy

Separate out the extra contextual info, naming the hash of the commit
being complained about, into a separate argument to $x->().  This
allows us to pass the message without that as an additional argument
to the callbacks.

This way we can still pass \&snag as a callback but other callers can
do something more sophisticated involving $cl.

No functional change for any existing callers.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>