Merge branch 'bc/diff-reject-empty-arg-to-pickaxe'

The -G/-S options to the "diff" family of commands caused us to hit
a BUG() when they get no values; they have been corrected.

* bc/diff-reject-empty-arg-to-pickaxe:
  diff: don't crash with empty argument to -G or -S

diff --cc diff.c
Simple merge