util: add sha256

Upcoming patches will teach host fingerprint checking to verify
non-SHA1 fingerprints.

Many x509 certificates these days are SHA-256. And modern browsers
often display the SHA-256 fingerprint for certificates. Since
SHA-256 fingerprints are highly visible and easy to obtain, we
want to support them for fingerprint pinning. So add SHA-256
support to util.

I did not add SHA-256 to DIGESTS and DIGESTS_BY_STRENGTH because
this will advertise the algorithm on the wire protocol. I wasn't
sure if that would be appropriate. I'm playing it safe by leaving
it out for now.

mercurial/util.py

@@ -65,6 +65,7 b' else:'
 
 md5 = hashlib.md5
 sha1 = hashlib.sha1
+sha256 = hashlib.sha256
 sha512 = hashlib.sha512
 _ = i18n._