merge: convert empty tree constant to the_hash_algo

To avoid dependency on a particular hash algorithm, convert a use of
EMPTY_TREE_SHA1_HEX to use the_hash_algo->empty_tree instead.  Since
both branches now use oid_to_hex, condense the if statement into a
ternary.

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