typing: add an assertion to mercurial/hgweb/webcommands.py to help pytype

I have no idea why this is needed, given the check immediately above.  But
without it, I'm getting this:

    File "/mnt/c/Users/Matt/hg/mercurial/hgweb/webcommands.py", line 1164, in filelog: Missing parameter 'toline' in call to function mercurial.dagop.blockdescendants [missing-parameter]
             Expected: (fctx, fromline, toline)
      Actually passed: (fctx, fromline)
    File "/mnt/c/Users/Matt/hg/mercurial/hgweb/webcommands.py", line 1166, in filelog: Missing parameter 'toline' in call to function mercurial.dagop.blockancestors [missing-parameter]
             Expected: (fctx, fromline, toline, followfirst)
      Actually passed: (fctx, fromline)

Differential Revision: https://phab.mercurial-scm.org/D10211

mercurial/hgweb/webcommands.py

@@ -1156,6 +1156,7 b' def filelog(web):'
 
     linerange = None
     if lrange is not None:
+        assert lrange is not None  # help pytype (!?)
         linerange = webutil.formatlinerange(*lrange)
         # deactivate numeric nav links when linerange is specified as this
         # would required a dedicated "revnav" class