trydiff: make filenames None when they don't exist...
trydiff: make filenames None when they don't exist
f1 and f2 are currently set always set to some filename, even for
added or deleted files. Let's instead set them to None to indicate
that one side of the diff doesn't exist. This lets us use the filename
variables instead of the content variables and simplify a bit since
the empty string is not a valid filename. More importantly, it paves
the way for further simplifications.