branchcache: rename itervalues() to iterheads() The itervalues() exists because branchcache() had a dict interface. Since it no longer has a dict interface, it makes sense to have better function names. If a person does not understand how branchcache stores info, it will be hard for them to guess what itervalues() does. Differential Revision: https://phab.mercurial-scm.org/D6152