changegroup: rename mfs to manifests In the name of readability. Differential Revision: https://phab.mercurial-scm.org/D4271

changegroup: clean up changelog callback changelog.read() calls changelog.changelogrevion() then turns it into a tuple. Let's call changelogrevision() directly and used named attributes to make code easier to read. While we're here, also change some variable names to make code easier to read. Differential Revision: https://phab.mercurial-scm.org/D4270