[C++] Fix comment_in_command.mk

Newline characters were escaped for some reason.
diff --git a/strutil_test.cc b/strutil_test.cc
index ce7e012..4d31c51 100644
--- a/strutil_test.cc
+++ b/strutil_test.cc
@@ -127,6 +127,7 @@
   ASSERT_EQ(EscapeShell("foo$`\\baz\"bar"), "foo\\$\\`\\\\baz\\\"bar");
   ASSERT_EQ(EscapeShell("$$"), "\\$$");
   ASSERT_EQ(EscapeShell("$$$"), "\\$$\\$");
+  ASSERT_EQ(EscapeShell("\\\n"), "\\\\\n");
 }
 
 }  // namespace