correct type of EMPTY_TREE_SHA1_BIN

correct type of EMPTY_TREE_SHA1_BIN

Functions such as hashcmp that expect a binary SHA-1 value take
parameters of type "unsigned char *" to avoid accepting a textual
SHA-1 passed by mistake.  Unfortunately, this means passing the string
literal EMPTY_TREE_SHA1_BIN requires an ugly cast.  Tweak the
definition of EMPTY_TREE_SHA1_BIN to produce a value of more
convenient type.

In the future the definition might change to

	extern const unsigned char empty_tree_sha1_bin[20];
	#define EMPTY_TREE_SHA1_BIN empty_tree_sha1_bin

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>