hgweb: simplify now that we always have a single path
Both `templatedir()` and `web.templatepath` are now always a single
path (or None).
Differential Revision: https://phab.mercurial-scm.org/D8788
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h