Currently, git's filter support does the opposite behaviour

(only includes files that match all exclude_patterns).
A new module, rbtools.utils.diffs, specifies helper functions

filename_match_any_patterns, which determines if a filename matches

at least one of the given patterns, and

remove_filenames_matching_patterns, which returns an iterable of the

filenames that do not match any given pattern.
The parameters to git diff and git diff-tree have been

reordered so that the revision specification is always the last

argument before the filenames.

All unit tests pass now.

Generated diffs with rbt diff in a git repository with more than

one file changed. When -X filename was specified, only that

file's diff was excluded from the output.