mingw: refuse paths containing reserved names

There are a couple of reserved names that cannot be file names on
Windows, such as `AUX`, `NUL`, etc. For an almost complete list, see
https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

If one would try to create a directory named `NUL`, it would actually
"succeed", i.e. the call would return success, but nothing would be
created.

Worse, even adding a file extension to the reserved name does not make
it a valid file name. To understand the rationale behind that behavior,
see https://devblogs.microsoft.com/oldnewthing/20031022-00/?p=42073

Let's just disallow them all.

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

t/t0060-path-utils.sh

@@ -465,11 +465,14 @@ test_expect_success 'match .gitmodules' '
 '
 
 test_expect_success MINGW 'is_valid_path() on Windows' '
-       test-tool path-utils is_valid_path \
+	test-tool path-utils is_valid_path \
 		win32 \
 		"win32 x" \
 		../hello.txt \
 		C:\\git \
+		comm \
+		conout.c \
+		lptN \
 		\
 		--not \
 		"win32 "  \
@@ -477,7 +480,13 @@ test_expect_success MINGW 'is_valid_path() on Windows' '
 		"win32."  \
 		"win32 . ." \
 		.../hello.txt \
-		colon:test
+		colon:test \
+		"AUX.c" \
+		"abc/conOut\$  .xyz/test" \
+		lpt8 \
+		"lpt*" \
+		Nul \
+		"PRN./abc"
 '
 
 test_done