githooks.txt: replace mentions of SHA-1 specific properties

githooks.txt: replace mentions of SHA-1 specific properties

The githooks(5) documentation states in several places that the hook
will receive a SHA-1 or hashes of 40 characters length. Given that we're
transitioning to a world where both SHA-1 and SHA-256 are supported,
this is inaccurate.

Fix the issue by replacing mentions of SHA-1 with "object name" and not
explicitly mentioning the hash size.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>