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