trailer: teach iterator about non-trailer lines

trailer: teach iterator about non-trailer lines

Previously the iterator did not iterate over non-trailer lines. This was
somewhat unfortunate, because trailer blocks could have non-trailer
lines in them since 146245063e (trailer: allow non-trailers in trailer
block, 2016-10-21), which was before the iterator was created in
f0939a0eb1 (trailer: add interface for iterating over commit trailers,
2020-09-27).

So if trailer API users wanted to iterate over all lines in a trailer
block (including non-trailer lines), they could not use the iterator and
were forced to use the lower-level trailer_info struct directly (which
provides a raw string array that includes all lines in the trailer
block).

Change the iterator's behavior so that we also iterate over non-trailer
lines, instead of skipping over them. The new "raw" member of the
iterator allows API users to access previously inaccessible non-trailer
lines. Reword the variable "trailer" to just "line" because this
variable can now hold both trailer lines _and_ non-trailer lines.

The new "raw" member is important because anyone currently not using the
iterator is using trailer_info's raw string array directly to access
lines to check what the combined key + value looks like. If we didn't
provide a "raw" member here, iterator users would have to re-construct
the unparsed line by concatenating the key and value back together again

Signed-off-by: Junio C Hamano <gitster@pobox.com>