sha1_name: convert GET_SHA1* flags to GET_OID*

sha1_name: convert GET_SHA1* flags to GET_OID*

Convert the flags for get_oid_with_context and friends to use "OID"
instead of "SHA1" in their names.

This transform was made by running the following one-liner on the
affected files:

  perl -pi -e 's/GET_SHA1/GET_OID/g'

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