If no `-o description=` is provided, fill it in automatically from the
first commit, just like title. Also allow filling in either, and
specifying them independently.
This means that `git push origin HEAD:refs/for/main/my-local-branch`
will fill in the PR title, *and* the description, without having to
specify additional parameters.
The description is the first commit's message without the first two
lines (the title and a newline, as customary).
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
259ad5d614