largefiles: make cat on standins do something
cat of a standin would silently fail.
The use of standins is mostly an implementation detail, but it is already a bit
leaking. Being able to see the content of standins might be convenient for
debugging.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.