builtin/verify-pack: implement an --object-format option

builtin/verify-pack: implement an --object-format option

A recently added test in t5702 started using git verify-pack outside of
a repository.  While this poses no problems with SHA-1, with SHA-256 we
implicitly rely on the setup of the repository to initialize our hash
algorithm settings.

Since we're not in a repository here, we need to provide git verify-pack
help to set things up properly.  git index-pack already knows an
--object-format option, so let's accept one as well and pass it down to
our git index-pack invocation.  Since we're now dynamically adjusting
the elements in argv, let's switch to using struct argv_array to manage
them.  Finally, let's make t5702 pass the proper argument on down to its
git verify-pack caller.

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