vendor/diffutils: upgrade from 3.3 to 3.7

vendor/diffutils: upgrade from 3.3 to 3.7

Summary of notable changes:

- new options --color and --palette to generate and configure colored
  output, compatible with less -R
- when binary files differ, diff now exits with status 1 as POSIX requires
- unless the --ignore-file-name-case option is used, diff now
  considers file names to be equal only if they are byte-for-byte
  equivalent
- diff -B no longer generates incorrect output if the two inputs each end
  with a one-byte incomplete line
- diff --brief no longer reports a difference for unusual identical files
- diff3 no longer malfunctions due to use-after-free
- using an invalid regular expression with --ignore-matching-lines=RE (-I)
  no longer causes stack overflow
- possible uninitialized memory read with --strip-trailing-cr fixed
- default algorithm has been adjusted to output higher-quality results,
  and further tweaked to deal better with larger files

For a complete history of changes see:

http://git.savannah.gnu.org/cgit/diffutils.git/log/