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