subtree: fix argument handling in check_parents

subtree: fix argument handling in check_parents

315a84f9aa0 (subtree: use commits before rejoins for splits, 2018-09-28)
changed the signature of check_parents from 'check_parents [REV...]'
to 'check_parents PARENTS_EXPR INDENT'. In other words the variable list
of parent revisions became a list embedded in a string. However it
neglected to unpack the list again before sending it to cache_miss,
leading to incorrect calls whenever more than one parent was present.
This is the case whenever a merge commit is processed, with the end
result being a loss of performance from unecessary rechecks.

The indent parameter was subsequently removed in e9525a8a029 (subtree:
have $indent actually affect indentation, 2021-04-27), but the argument
handling bug remained.

For consistency, take multiple arguments in check_parents,
and pass all of them to cache_miss separately.

Signed-off-by: James Limbouris <james@digitalmatter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>