clone: don't include empty revlogs in stream

serve: don't send any content headers with 304 responses Fixes HTTP protocol violation introduced in cf5c76017e11. 'hg serve' would show a stacktrace when loading pages that not had been modified. There was test coverage for this, but the wrong response headers wasn't shown and thus not detected.

tests: fix doctest stability over Python versions pprint ain't pretty in Python 2.4: Changed in version 2.5: Dictionaries are sorted by key before the display is computed; before 2.5, a dictionary was sorted only if its display required more than one line, although that wasn’t documented. Fixes issue introduced in 404feac78b8a.