gnupg: discover hash algorithm used for signature

$ echo foo | gpg --detach-sign --armor | LC_ALL=C gpg --list-packets --verbose

gpg: armor header: Version: GnuPG v1
:signature packet: algo 17, keyid 14C85A0824EAF823
	version 4, created 1440053810, md5len 0, sigclass 0x00
	digest algo 2, begin of digest 59 54
	hashed subpkt 2 len 4 (sig created 2015-08-20)
	subpkt 16 len 8 (issuer key ID 14C85A0824EAF823)
	data: [160 bits]
	data: [158 bits]