Because it's a special character on Windows ("c:").
Use same technique already used for '/' and '\'.
I didn't record how I generated their encoded forms before, so am sure
there was a better way, but the way I did it now is to look at
ghci> encodeFilePath "∕"
"\226\136\149"
And then the difference from that to "\56546\56456\56469"
is adding 56320 to each, to get up to the escaped code plane.
See comment for why I think handling ':' is ok, but that other illegal
windows filenames won't. Note that, this should be enough to make the
test suite always work. Other windows illegal filenames will fail at
checkout time when it tries to put the illegal filename on the
filesystem.
