tests (pack-objects): use the full, unabbreviated `--revs` option

tests (pack-objects): use the full, unabbreviated `--revs` option

To use the singular form of a word, when the option wants the plural
form (and quietly expands it because it thinks it was abbreviated), is
an easy mistake to make, and t5317 contains almost two dozen of them.

However, using abbreviated options in tests is a bit fragile, so we will
disallow use of abbreviated options in our test suite.

In preparation for this change, let's fix
`t5317-pack-objects-filter-objects.sh`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>