hgext: replace references to hashlib.sha1 with hashutil.sha1
When in a non-pure build of Mercurial, this will provide protections
against SHA1 collision attacks.
Differential Revision: https://phab.mercurial-scm.org/D7851
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed