repack: add --name-hash-version option

repack: add --name-hash-version option

The new '--name-hash-version' option for 'git repack' is a simple
pass-through to the underlying 'git pack-objects' subcommand. However,
this subcommand may have other options and a temporary filename as part
of the subcommand execution that may not be predictable or could change
over time.

The existing test_subcommand method requires an exact list of arguments
for the subcommand. This is too rigid for our needs here, so create a
new method, test_subcommand_flex. Use it to check that the
--name-hash-version option is passing through.

Since we are modifying the 'git repack' command, let's bring its usage
in line with the Documentation's synopsis. This removes it from the
allow list in t0450 so it will remain in sync in the future.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>