t1300: mark all test cases with funny filenames as !MINGW

On Windows, it is impossible to create a file whose name contains a
quote character. We already excluded test cases using such files from
running on Windows when git.exe itself was tested.

However, we still had two test cases that try to create such a file, and
redirect stdin from such a file, respectively. This *seems* to work in
Git for Windows' Bash due to an obscure feature inherited from Cygwin:
illegal filename characters are simply mapped into/from a private UTF-8
page. Pure Win32 programs (such as git.exe) *still* cannot work with
those files, of course, but at least Unix shell scripts pretend to be
able to.

This entire strategy breaks down when switching to any Unix shell
lacking support for that private UTF-8 page trick, e.g. BusyBox-w32's
ash. So let's just exclude test cases that test whether the Unix shell
can redirect to/from files with "funny names" those from running on
Windows, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t1300-config.sh

@@ -1709,7 +1709,7 @@ test_expect_success '--show-origin getting a single key' '
 	test_cmp expect output
 '
 
-test_expect_success 'set up custom config file' '
+test_expect_success !MINGW 'set up custom config file' '
 	CUSTOM_CONFIG_FILE="file\" (dq) and spaces.conf" &&
 	cat >"$CUSTOM_CONFIG_FILE" <<-\EOF
 		[user]
@@ -1725,7 +1725,7 @@ test_expect_success !MINGW '--show-origin escape special file name characters' '
 	test_cmp expect output
 '
 
-test_expect_success '--show-origin stdin' '
+test_expect_success !MINGW '--show-origin stdin' '
 	cat >expect <<-\EOF &&
 		standard input:	user.custom=true
 	EOF