Add a test to verify FILENAME_MAX

Since we now rely on it (at least under Solaris), I figured we probably
want to make sure it's accurate. The new test makes sure that creating a
file with a name of length FILENAME_MAX+1 fails.

tests-clar/core/dirent.c

@@ -222,3 +222,14 @@ void test_core_dirent__traverse_weird_filenames(void)
 
 	check_counts(&odd);
 }
+
+/* test filename length limits */
+void test_core_dirent__length_limits(void)
+{
+	char *big_filename = (char *)git__malloc(FILENAME_MAX + 1);
+	memset(big_filename, 'a', FILENAME_MAX + 1);
+	big_filename[FILENAME_MAX] = 0;
+
+	cl_must_fail(p_creat(big_filename, 0666));
+	git__free(big_filename);
+}