tests: cover revision conversion logic in githelp tests There was no test involving actual conversion of option values when they contain a git revision name (to be converted as a hg one by hgext.githelp.convert()). Adding one. This test would fail on Python 3 without https://phab.mercurial-scm.org/D7537. Differential Revision: https://phab.mercurial-scm.org/D7540