builtin-am: rerere support

builtin-am: rerere support

git-am.sh will call git-rerere at the following events:

* "git rerere" when a three-way merge fails to record the conflicted
  automerge results. Since 8389b52 (git-rerere: reuse recorded resolve.,
  2006-01-28)

  * Since cb6020b (Teach --[no-]rerere-autoupdate option to merge,
    revert and friends, 2009-12-04), git-am.sh supports the
    --[no-]rerere-autoupdate option as well, and would pass it to
    git-rerere.

* "git rerere" when --resolved, to record the hand resolution. Since
  f131dd4 (rerere: record (or avoid misrecording) resolved, skipped or
  aborted rebase/am, 2006-12-08)

* "git rerere clear" when --skip-ing. Since f131dd4 (rerere: record (or
  avoid misrecording) resolved, skipped or aborted rebase/am,
  2006-12-08)

* "git rerere clear" when --abort-ing. Since 3e5057a (git am --abort,
  2008-07-16)

Re-implement the above in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>