builtin-am: implement committing applied patch

builtin-am: implement committing applied patch

Implement do_commit(), which commits the index which contains the
results of applying the patch, along with the extracted commit message
and authorship information.

Since 29b6754 (am: remove rebase-apply directory before gc, 2010-02-22),
git gc --auto is also invoked to pack the loose objects that are created
from making the commits.

Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>