Merge branch 'nd/diff-parseopt'

A brown-paper-bag bugfix to a change already in 'master'.

* nd/diff-parseopt:
  parse-options: check empty value in OPT_INTEGER and OPT_ABBREV
  diff-parseopt: restore -U (no argument) behavior
  diff-parseopt: correct variable types that are used by parseopt

t/t4013-diff-various.sh

@@ -338,6 +338,8 @@ format-patch --inline --stdout initial..master^^
 format-patch --stdout --cover-letter -n initial..master^
 
 diff --abbrev initial..side
+diff -U initial..side
+diff -U1 initial..side
 diff -r initial..side
 diff --stat initial..side
 diff -r --stat initial..side