rebase: cleanup action handling

rebase: cleanup action handling

Treating the action as a string is a hang over from the scripted
rebase. The last commit removed the only remaining use of the action
that required a string so lets convert the other action users to use
the existing action enum instead. If we ever need the action name as a
string in the future the action_names array exists exactly for that
purpose.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>