browser: sort names that differ only in case with uppercase first

This is the opposite of what 'ls' does in a UTF-8 locale, but nano
has never followed the collating rules of Unicode (uppercase after
lowercase, ignoring punctuation, and so on) -- it would be strange
to change that now.

Until now, nano left such equivalent names unsorted, in a seemingly
random order.

This fixes https://savannah.gnu.org/bugs/?59059.

Bug existed since before version 2.0.6.

src/files.c

@@ -2354,11 +2354,13 @@ int diralphasort(const void *va, const void *vb)
 	if (!aisdir && bisdir)
 		return 1;
 
-	/* Standard function brain damage: We should be sorting
-	 * alphabetically and case-insensitively according to the current
-	 * locale, but there's no standard strcasecoll() function, so we
-	 * have to use multibyte strcasecmp() instead. */
-	return mbstrcasecmp(a, b);
+	int difference = mbstrcasecmp(a, b);
+
+	/* If two names are equivalent when ignoring case, compare them bytewise. */
+	if (difference == 0)
+		return strcmp(a, b);
+	else
+		return difference;
 }
 #endif