fix(args): -o and -p are not allowed together

setting prepend and output together will ignore the prepend flag

Signed-off-by: Anton Engelhardt <antoncengelhardt@icloud.com>

git-cliff/src/lib.rs

@@ -386,6 +386,11 @@ pub fn run(mut args: Opt) -> Result<()> {
 			)));
 		}
 	}
+	if args.output.is_some() && args.prepend.is_some() {
+		return Err(Error::ArgumentError(String::from(
+			"'-o' and '-p' cannot be used together",
+		)));
+	}
 	if args.body.is_some() {
 		config.changelog.body.clone_from(&args.body);
 	}