"git update-index --index-version N" is used to set the index format
version to a specific version, but there was no way to query the
current version used in the on-disk index file.
Teach the command a new "--show-index-version" option, and also
teach the "--index-version N" option to report what the version was
when run with the "--verbose" option.
Helped-by: Linus Arver <linusa@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>