hgweb: enumerate lines in loop header, not before
Doing this will allow access to the lines in arbitrary order (because the
result of enumerate() is an iterator), and that will help calculating rowspan
for annotate blocks.
sslutil: add assertion to prevent accidental CA usage on Windows
Yuya suggested we add this check to ensure we don't accidentally try
to load user-writable paths on Windows if we change the control
flow of this function later.