templatefilters: rename commonprefix to commondir Two reasons: * It makes it clearer that it's not a generic common string prefix (e.g. commonprefix(["bar", "baz"]) is not "ba", but "") * If we ever want a filter for generic common string prefix, then the name is now available for that. "commondir" does not describe the prefix-ness, however. I'm happy to rename it "commondirprefix" or "commonprefixdir" if others prefer. Differential Revision: https://phab.mercurial-scm.org/D3731