mingw: skip some tests in t9115 due to file name issues

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: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9115-git-svn-dcommit-funky-renames.sh

@@ -93,7 +93,7 @@ test_expect_success 'git svn rebase works inside a fresh-cloned repository' '
 # > to special UNICODE characters in the range 0xf000 to 0xf0ff (the
 # > "Private use area") when creating or accessing files.
 prepare_a_utf8_locale
-test_expect_success UTF8 'svn.pathnameencoding=cp932 new file on dcommit' '
+test_expect_success UTF8,!MINGW,!UTF8_NFD_TO_NFC 'svn.pathnameencoding=cp932 new file on dcommit' '
 	LC_ALL=$a_utf8_locale &&
 	export LC_ALL &&
 	neq=$(printf "\201\202") &&
@@ -105,7 +105,7 @@ test_expect_success UTF8 'svn.pathnameencoding=cp932 new file on dcommit' '
 '
 
 # See the comment on the above test for setting of LC_ALL.
-test_expect_success 'svn.pathnameencoding=cp932 rename on dcommit' '
+test_expect_success !MINGW,!UTF8_NFD_TO_NFC 'svn.pathnameencoding=cp932 rename on dcommit' '
 	LC_ALL=$a_utf8_locale &&
 	export LC_ALL &&
 	inf=$(printf "\201\207") &&