Merge branch 'jc/cache-unmerge'

Merge branch 'jc/cache-unmerge'

* jc/cache-unmerge:
  rerere forget path: forget recorded resolution
  rerere: refactor rerere logic to make it independent from I/O
  rerere: remove silly 1024-byte line limit
  resolve-undo: teach "update-index --unresolve" to use resolve-undo info
  resolve-undo: "checkout -m path" uses resolve-undo information
  resolve-undo: allow plumbing to clear the information
  resolve-undo: basic tests
  resolve-undo: record resolved conflicts in a new index extension section
  builtin-merge.c: use standard active_cache macros

Conflicts:
	builtin-ls-files.c
	builtin-merge.c
	builtin-rerere.c