prefer "hash mismatch" to "sha1 mismatch"

prefer "hash mismatch" to "sha1 mismatch"

To future-proof ourselves against a change in the hash, let's use the
more generic "hash mismatch" to refer to integrity problems. Note that
we do advertise this exact string in git-fsck(1). However, the message
itself is marked for translation, meaning we do not expect it to be
machine-readable.

While we're touching that documentation, let's also update it for
grammar and clarity.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>