Merge branch 'ab/pickaxe-pcre2'

Merge branch 'ab/pickaxe-pcre2'

Rewrite the backend for "diff -G/-S" to use pcre2 engine when
available.

* ab/pickaxe-pcre2: (22 commits)
  xdiff-interface: replace discard_hunk_line() with a flag
  xdiff users: use designated initializers for out_line
  pickaxe -G: don't special-case create/delete
  pickaxe -G: terminate early on matching lines
  xdiff-interface: allow early return from xdiff_emit_line_fn
  xdiff-interface: prepare for allowing early return
  pickaxe -S: slightly optimize contains()
  pickaxe: rename variables in has_changes() for brevity
  pickaxe -S: support content with NULs under --pickaxe-regex
  pickaxe: assert that we must have a needle under -G or -S
  pickaxe: refactor function selection in diffcore-pickaxe()
  perf: add performance test for pickaxe
  pickaxe/style: consolidate declarations and assignments
  diff.h: move pickaxe fields together again
  pickaxe: die when --find-object and --pickaxe-all are combined
  pickaxe: die when -G and --pickaxe-regex are combined
  pickaxe tests: add missing test for --no-pickaxe-regex being an error
  pickaxe tests: test for -G, -S and --find-object incompatibility
  pickaxe tests: add test for "log -S" not being a regex
  pickaxe tests: add test for diffgrep_consume() internals
  ...