builtin/show-index: provide options to determine hash algo

builtin/show-index: provide options to determine hash algo

show-index is capable of reading any possible index file whether or not
the index is inside a repository.  However, because our index files lack
metadata about the hash algorithm in use, it's not possible to
autodetect the algorithm that a particular index file is using.

In order to allow us to read index files of any algorithm, let's set up
the .git directory gently so that we default to the algorithm for the
current repository, and add an --object-format option to allow users to
override this setting and continue to run show-index outside of a
repository altogether.  Let's also document this new option so that
people can find it and use it.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>