manifest: disallow setting the node id of an entry to None
manifest.diff() uses None as a special value to denote the absence of
a file, so setting a file node to None means you then can't trust
manifest.diff().
This should also make future manifest work slightly easier.
#require test-repo pyflakes
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:**.py or grep("^!#.*python")' 2>/dev/null \
> | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
tests/filterpyflakes.py:58: undefined name 'undefinedname'