t6423: introduce Windows-specific handling for symlinking to /dev/null

The device `/dev/null` does not exist on Windows, it's called `NUL`
there. Calling `ln -s /dev/null my-symlink` in a symlink-enabled MSYS2
Bash will therefore literally link to a file or directory called `null`
that is supposed to be in the current drive's top-level `dev` directory.
Which typically does not exist.

The test, however, really wants the created symbolic link to point to
the NUL device. Let's instead use the `mklink` utility on Windows to
perform that job, and keep using `ln -s /dev/null <target>` on
non-Windows platforms.

While at it, add the missing `SYMLINKS` prereq because this test _still_
would not pass on Windows before support for symbolic links is
upstreamed from Git for Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6423-merge-rename-directories.sh

@@ -5158,13 +5158,18 @@ test_setup_12m () {
 		git switch B &&
 		git rm dir/subdir/file &&
 		mkdir dir &&
-		ln -s /dev/null dir/subdir &&
+		if test_have_prereq MINGW
+		then
+			cmd //c 'mklink dir\subdir NUL'
+		else
+			ln -s /dev/null dir/subdir
+		fi &&
 		git add . &&
 		git commit -m "B"
 	)
 }
 
-test_expect_success '12m: Change parent of renamed-dir to symlink on other side' '
+test_expect_success SYMLINKS '12m: Change parent of renamed-dir to symlink on other side' '
 	test_setup_12m &&
 	(
 		cd 12m &&