trailer: clarify failure modes in parse_trailer

trailer: clarify failure modes in parse_trailer

The parse_trailer function has a few modes of operation, all depending
on whether the separator is present in its input, and if yes, the
separator's position. Some of these modes are failure modes, and these
failure modes are handled differently depending on whether the trailer
line was sourced from a file or from a command-line argument.

Extract a function to find the separator, allowing the invokers of
parse_trailer to determine how to handle the failure modes instead of
making parse_trailer do it. In this function, also take in the list of
separators, so that we can distinguish between command line arguments
(which allow '=' as separator) and file input (which does not allow '='
as separator).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>