interpret-trailers: handle message without trailing newline

interpret-trailers: handle message without trailing newline

When git-interpret-trailers is used to add a trailer to a message that
does not end in a trailing newline, the new trailer is added on the line
immediately following the message instead of as a trailer block
separated from the message by a blank line.

For example, if a message's text was exactly "The subject" with no
trailing newline present, `git interpret-trailers --trailer
my-trailer=true` will result in the following malformed commit message:

    The subject
    my-trailer: true

While it is generally expected that a commit message should end with a
newline character, git-interpret-trailers should not be returning an
invalid message in this case.

Use `strbuf_complete_line` to ensure that the message ends with a
newline character when reading the input.

Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>