These two tests wanted to write file names which are incompatible with
Windows' file naming rules (even if they pass using Cygwin due to
Cygwin's magic path mangling).

While at it, skip the same tests also on MacOSX/HFS, as pointed out by
Torsten Bögershausen.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NTorsten Bögershausen <tboegi@web.de>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>