Merge branch 'tr/maint-word-diff-regex-sticky' into maint

The regexp configured with diff.wordregex was incorrectly reused
across files.

By Thomas Rast (2) and Johannes Sixt (1)
* tr/maint-word-diff-regex-sticky:
  diff: tweak a _copy_ of diff_options with word-diff
  diff: refactor the word-diff setup from builtin_diff_cmd
  t4034: diff.*.wordregex should not be "sticky" in --word-diff

diff --cc diff.c
Simple merge