osxkeychain: clarify that we ignore unknown lines

Like in all the other credential helpers, the osxkeychain helper
ignores unknown credential lines.

Add a comment (a la the other helpers) to make it clear and explicit
that this is the desired behaviour.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/credential/osxkeychain/git-credential-osxkeychain.c

@@ -159,6 +159,11 @@ static void read_credential(void)
 			username = xstrdup(v);
 		else if (!strcmp(buf, "password"))
 			password = xstrdup(v);
+		/*
+		 * Ignore other lines; we don't know what they mean, but
+		 * this future-proofs us when later versions of git do
+		 * learn new lines, and the helpers are updated to match.
+		 */
 	}
 }