fix unidiff headers for empty files

add a zero-context test and rename the -C option to -U (matches diff(1))

add a test case to trigger a bug where diff chunks grow way too much context

add support for ignoring whitespace (-w option) Tests grow the ability to call diff with options. Options to use are encoded in the test file name as a single dash followed by a string of option letters. E.g. -wp for 'ignore whitespace' and 'do patience'

switch default to unidiff

initial commit