builtin/history: generalize function to commit trees

builtin/history: generalize function to commit trees

The function `commit_tree_with_edited_message_ext()` can be used to
commit a tree with a specific list of parents with an edited commit
message. This function is useful outside of editing the commit message
though, as it also performs the plumbing to extract the original commit
message and strip some headers from it.

Refactor the function to receive a flags field that allows the caller to
control whether or not the commit message should be edited, or whether
it should be retained as-is. This will be used in a subsequent commit.

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