builtin-am: implement patch queue mechanism

git-am applies a series of patches. If the process terminates
abnormally, we want to be able to resume applying the series of patches.
This requires the session state to be saved in a persistent location.

Implement the mechanism of a "patch queue", represented by 2 integers --
the index of the current patch we are applying and the index of the last
patch, as well as its lifecycle through the following functions:

* am_setup(), which will set up the state directory
  $GIT_DIR/rebase-apply. As such, even if the process exits abnormally,
  the last-known state will still persist.

* am_load(), which is called if there is an am session in
  progress, to load the last known state from the state directory so we
  can resume applying patches.

* am_run(), which will do the actual patch application. After applying a
  patch, it calls am_next() to increment the current patch index. The
  logic for applying and committing a patch is not implemented yet.

* am_destroy(), which is finally called when we successfully applied all
  the patches in the queue, to clean up by removing the state directory
  and its contents.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Stefan Beller <sbeller@google.com>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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