allow non-trailers and multiple-line trailers

I've updated patch 5/8 to use strcspn and to pass in the list of separators, meaning that we no longer accept '=' in file input (and also updated its commit message accordingly).

We also discussed inlining find_separator, but after looking at the code, I think that it is more convenient if find_separator returns -1 when there is no separator, because of the 3 times it is used (as of 8/8), it is checked twice with '>= 1' (since both "no separator" and "string begins with separator" are handled in the same way - treating them as a non-trailer line). So I have left it as its own function.

No other updates.

Jonathan Tan (8):
trailer: improve const correctness
trailer: use list.h for doubly-linked list
trailer: streamline trailer item create and add
trailer: make args have their own struct
trailer: clarify failure modes in parse_trailer
trailer: allow non-trailers in trailer block
trailer: forbid leading whitespace in trailers
trailer: support values folded to multiple lines

 Documentation/git-interpret-trailers.txt |  14 +-
 t/t7513-interpret-trailers.sh            | 299 +++++++++++++++
 trailer.c                                | 620 +++++++++++++++++--------------
 3 files changed, 654 insertions(+), 279 deletions(-)

2.8.0.rc3.226.g39d4020