MinGW: fix diff --no-index /dev/null ...

When launching "diff --no-index" with a parameter "/dev/null", the MSys
bash converts the "/dev/null" to a "nul", which usually makes sense.  But
diff --no-index got confused and tried to access a _file_ called "nul".

While at it, the comment in t4012, expressed as ":# <text>" was turned
into ": <text>" so that MSys' path name mangling does not kick in.

With this patch, t4012 passes in msysGit.

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

diff-no-index.c

@@ -38,6 +38,10 @@ static int get_mode(const char *path, int *mode)
 
 	if (!path || !strcmp(path, "/dev/null"))
 		*mode = 0;
+#ifdef _WIN32
+	else if (!strcasecmp(path, "nul"))
+		*mode = 0;
+#endif
 	else if (!strcmp(path, "-"))
 		*mode = create_ce_mode(0666);
 	else if (lstat(path, &st))

t/t4012-diff-binary.sh

@@ -87,7 +87,7 @@ nul_to_q() {
 
 test_expect_success 'diff --no-index with binary creation' '
 	echo Q | q_to_nul >binary &&
-	(:# hide error code from diff, which just indicates differences
+	(: hide error code from diff, which just indicates differences
 	 git diff --binary --no-index /dev/null binary >current ||
 	 true
 	) &&