At the moment, this command runs forever before noticing that the
svn:mime-type is binary, and prints "cannot display, file marked as
binary type." I mean, it's still useful information that the binary
files differ... but we can be smarter and faster:
Instead, use the svn_wc_text_changed_p tricks: look at timestamps and
filesizes for a possible quick answer. If timestamps are identical,
there's no diff. If filesizes are different, there's definitely a
diff, but look at the mimetype before trying to compute the diff;
there's no point in computing the diff if you can't even display it.
Improvement
Critical
SVN-1574 'svn diff' leak on large files
