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.
$ "$TESTDIR/hghave" execbit || exit 80
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..