contrib: restore the hg fix configuration in the examples
After decc3bd3f20d, running black will DTRT, but running hg fix did nothing
(unless the example config file was %included, in which case it truncated the
file instead of formatting it). I'm not sure why that was happening, but let's
not leave a code shredder laying around.
Differential Revision: https://phab.mercurial-scm.org/D10311