util: use pycompat.bytestr() instead of str() This fixes at least some environment variable prints for util.system() callers on Python 3. Yay! Differential Revision: https://phab.mercurial-scm.org/D2434

showconfig: allow multiple section.name selectors (issue5797) This seems useful and we can disambiguate the output format solely by the type of the command arguments.

showconfig: use set to filter sections and entry names Before, an entry matching the specified section could be printed twice if the selector wasn't unique. "sections" and "items" are renamed because it's hard to distinguish "sections" from the loop variable "section".