hash: require hash algorithm in `oidread()` and `oidclr()`

hash: require hash algorithm in `oidread()` and `oidclr()`

Both `oidread()` and `oidclr()` use `the_repository` to derive the hash
function that shall be used. Require callers to pass in the hash
algorithm to get rid of this implicit dependency.

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