[PATCH v4 0/8] allow non-trailers and multiple-line trailers

Main changes are:
 - implemented the previously discussed trailer block recognizing rule
   (recognized trailer + 25% trailers or 100% trailers)
 - forbidding leading whitespace in trailers to avoid false positives

Once the recognized trailer + 25% trailers rule is implemented, implementing the 100% trailer rule gives us backwards compatibility and is only a few lines of code, so I included it.

Summary of changes from v3:
 2/6->2/8:
   - squashed Ramsay Jones's "static" patch
 new->5/8:
   - new patch
 5/6->6/8:
   - new trailer block recognizing rule
   - reverted to the existing behavior of ignoring comments, since the
     number of trailers and non-trailers in the trailer block now
     matters more
 new->7/8:
   - new patch
 6/6->8/8:
   - updated trailer block recognizing code, since the continuation
     lines must not be counted if they follow a trailer line

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                                | 619 +++++++++++++++++--------------
 3 files changed, 651 insertions(+), 281 deletions(-)