Use legacy hash for legacy formats

Use legacy hash for legacy formats

We have a large variety of data formats and protocols where no hash
algorithm was defined and the default was assumed to always be SHA-1.
Instead of explicitly stating SHA-1, let's use the constant to represent
the legacy hash algorithm (which is still SHA-1) so that it's clear
for documentary purposes that it's a legacy fallback option and not an
intentional choice to use SHA-1.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>