Merge branch 'en/diffstat-utf8-truncation-fix'

The computation to shorten the filenames shown in diffstat measured
width of individual UTF-8 characters to add up, but forgot to take
into account error cases (e.g., an invalid UTF-8 sequence, or a
control character).

* en/diffstat-utf8-truncation-fix:
  diff: fix out-of-bounds reads and NULL deref in diffstat UTF-8 truncation