Merge branch 'jc/apply-3way'

Merge branch 'jc/apply-3way'

"git apply" learned to wiggle the base version and perform three-way
merge when a patch does not exactly apply to the version you have.

* jc/apply-3way:
  apply: tests for the --3way option
  apply: document --3way option
  apply: allow rerere() to work on --3way results
  apply: register conflicted stages to the index
  apply: --3way with add/add conflict
  apply: move verify_index_match() higher
  apply: plug the three-way merge logic in
  apply: fall back on three-way merge
  apply: accept -3/--3way command line option
  apply: move "already exists" logic to check_to_create()
  apply: move check_to_create_blob() closer to its sole caller
  apply: further split load_preimage()
  apply: refactor "previous patch" logic
  apply: split load_preimage() helper function out
  apply: factor out checkout_target() helper function
  apply: refactor read_file_or_gitlink()
  apply: clear_image() clears things a bit more
  apply: a bit more comments on PATH_TO_BE_DELETED
  apply: fix an incomplete comment in check_patch()