Merge branch 'mm/diff-U-takes-no-negative-values'

The command line parser for "git diff" learned a few options take
only non-negative integers.

* mm/diff-U-takes-no-negative-values:
  parse-options: clarify what "negated" means for PARSE_OPT_NONEG
  xdiff: guard against negative context lengths
  diff: reject negative values for -U/--unified
  diff: reject negative values for --inter-hunk-context

diff --cc diff.c
Simple merge

diff --cc t/t4032-diff-inter-hunk-context.sh
Simple merge