reset_head(): remove action parameter

reset_head(): remove action parameter

The only use of the action parameter is to setup the error messages
for unpack_trees(). All but two cases pass either "checkout" or
"reset". The case that passes "reset --hard" would be better passing
"reset" so that the error messages match the builtin reset command
like all the other callers that are doing a reset. The case that
passes "Fast-forwarded" is only updating HEAD and so the parameter is
unused in that case as it does not call unpack_trees(). The value to
pass to setup_unpack_trees_porcelain() can be determined by checking
whether flags contains RESET_HEAD_HARD without the caller having to
specify it.

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