"git rerere" can encounter two or more files with the same conflict
signature that have to be resolved in different ways, but there was
no way to record these separate resolutions.

* jc/rerere-multi:
  rerere: adjust 'forget' to multi-variant world order
  rerere: split code to call ll_merge() further
  rerere: move code related to "forget" together
  rerere: gc and clear
  rerere: do use multiple variants
  t4200: rerere a merge with two identical conflicts
  rerere: allow multiple variants to exist
  rerere: delay the recording of preimage
  rerere: handle leftover rr-cache/$ID directory and postimage files
  rerere: scan $GIT_DIR/rr-cache/$ID when instantiating a rerere_id
  rerere: split conflict ID further