Merge branch 'rs/pickaxe-i'

Allow the options -i/--regexp-ignore-case, --pickaxe-regex, and -S
to be used together and work as expected to perform a pickaxe
search using case-insensitive regular expression matching.

* rs/pickaxe-i:
  pickaxe: simplify kwset loop in contains()
  pickaxe: call strlen only when necessary in diffcore_pickaxe_count()
  pickaxe: move pickaxe() after pickaxe_match()
  pickaxe: merge diffcore_pickaxe_grep() and diffcore_pickaxe_count() into diffcore_pickaxe()
  pickaxe: honor -i when used with -S and --pickaxe-regex
  t4209: use helper functions to test --author
  t4209: use helper functions to test --grep
  t4209: factor out helper function test_log_icase()
  t4209: factor out helper function test_log()
  t4209: set up expectations up front