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: NJunio C Hamano <gitster@pobox.com>
Helped-by: NStefan Beller <sbeller@google.com>
Helped-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NPaul Tan <pyokagan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

For the purpose of rewriting git-am.sh into a C builtin, implement a
skeletal builtin/am.c that redirects to $GIT_EXEC_PATH/git-am if the
environment variable _GIT_USE_BUILTIN_AM is not defined. Since in the
Makefile git-am.sh takes precedence over builtin/am.c,
$GIT_EXEC_PATH/git-am will contain the shell script git-am.sh, and thus
this allows us to fall back on the functional git-am.sh when running the
test suite for tests that depend on a working git-am implementation.

Since git-am.sh cannot handle any environment modifications by
setup_git_directory(), "am" is declared with no setup flags in git.c. On
the other hand, to re-implement git-am.sh in builtin/am.c, we need to
run all the git dir and work tree setup logic that git.c typically does
for us. As such, we work around this temporarily by copying the logic in
git.c's run_builtin(), which is roughly:

	prefix = setup_git_directory();
	trace_repo_setup(prefix);
	setup_work_tree();

This redirection should be removed when all the features of git-am.sh
have been re-implemented in builtin/am.c.
Helped-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NPaul Tan <pyokagan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>