fix(args): prepend and output only compatible if paths are different

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

git-cliff/src/lib.rs

@@ -386,11 +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.output.is_some() && args.prepend.is_some() && args.output.as_ref() == args.prepend.as_ref() {
+        return Err(Error::ArgumentError(String::from(
+            "'-o' and '-p' can only be used together if they point to different files",
+        )));
+    }
 	if args.body.is_some() {
 		config.changelog.body.clone_from(&args.body);
 	}

website/docs/usage/examples.md

@@ -72,7 +72,7 @@ Prepend new changes to an existing changelog file:
 ```bash
 # 1- changelog header is removed from CHANGELOG.md
 # 2- new entries are prepended to CHANGELOG.md without footer part
-# the --prepend option is incompatible with -o (output)
+# the --prepend option is incompatible with -o (output) if the file paths are equal
 git cliff --unreleased --tag 1.0.0 --prepend CHANGELOG.md
 ```