httppeer: improve protocol check
Pre-0.6c hgweb used text/plain for protocol responses. This meant
that a web server could serve a static file and confuse a client into
generating a nasty traceback.
Now we insist that text/plain protocol responses not include a
Content-Length, which older hgweb didn't generate but will typically
be produced for static files.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown