tests: add test for remotefilelog interactions with hgweb
It's not uncommon for hg users to rely on hgweb as a simple GUI and
history browser (I do this all the time on Mercurial), but we lack any
tests to ensure things keep working.
At present, this merely demonstrates the "view contents of a single
file" endpoint is broken. I'll fix that in a subsequent change.
Differential Revision: https://phab.mercurial-scm.org/D8298