builtin/history: split out extended function to create commits

builtin/history: split out extended function to create commits

In the next commit we're about to introduce a new command that splits up
a commit into two. Most of the logic will be shared with rewording
commits, except that we also need to have control over the parents and
the old/new trees.

Extract a new function `commit_tree_with_edited_message_ext()` to
prepare for this commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>