rebase -i: simplify get_revision_ranges()

rebase -i: simplify get_revision_ranges()

Now that all the external users of head_hash have been converted to
use a opts->orig_head instead we can stop returning head_hash from
get_revision_ranges().

Because we want to pass the full object names back to the caller in
`revisions` the find_unique_abbrev_r() call that was used to initialize
`head_hash` is replaced with oid_to_hex().

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