sha1-file: add functions for hex empty tree and blob OIDs

sha1-file: add functions for hex empty tree and blob OIDs

Oftentimes, we'll want to refer to an empty tree or empty blob by its
hex name without having to call oid_to_hex or explicitly refer to
the_hash_algo.  Add helper functions that format these values into
static buffers and return them for easy use.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>