merge: split reduce_parents() out of collect_parents()

merge: split reduce_parents() out of collect_parents()

The latter does two separate things:

 - Parse the list of commits on the command line, and formulate the
   list of commits to be merged (including the current HEAD);

 - Compute the list of parents to be recorded in the resulting merge
   commit.

Split the latter into a separate helper function, so that we can
later supply the list commits to be merged from a different source
(namely, FETCH_HEAD).

Signed-off-by: Junio C Hamano <redacted>