templatekw: fix dict construction in _showlist to not mix bytes and strs
What we had was fine on Python 2, but was slightly wrong on Python
3. This works on both.
Differential Revision: https://phab.mercurial-scm.org/D2554
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program