git_message_trailers: it would be useful to get both subject+body and trailers (for modifying commit messages)

[LemmingAvalanche]

git_message_trailers is great for querying the trailers. But what if you want to:

1. Search for one of the trailers
2. Delete it
3. Make a new message (subject, body, trailers) from that updated trailers data structure

How do you accomplish (3) if you don’t want to re-parse the message in order to separate subject+body from the trailers?

It would be convenient if libgit2 provided the following:

1. A function which populates two pointers: one to the string representing subject+body and a pointer to the same output as git_message_trailers
2. A function which takes the two pointers above and combines them back (if it assumes that both are valid then I guess it would just consist of adding a linefeed between the two parts)

Other thoughts

It seems that git_message_trailers indeed deals not only with a regular commit message but also with git format-patch-type output, considering that it looks for the divider (---) which separates the whole commit message (subject+body+trailers) from the patch. But just dealing with commit trailers (i.e. not necessarily also the optional patch part) would be perfectly fine… at least for me. :)