fix: match GNU error format for unrecognized options (#180)

* fix: match GNU error format for unrecognized options

Use single quotes and remove colon to match GNU diff/cmp output:
`unrecognized option '--foobar'` instead of `unrecognized option: "--foobar"`

Also use `contains` instead of `starts_with` in the integration test
to handle the command prefix (e.g. `cmp: unrecognized option ...`).

Follow-up to #178 / #179.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply cargo fmt formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Ryujiyasu

src/cmp.rs

@@ -217,7 +217,7 @@ pub fn parse_params<I: Iterator<Item = OsString>>(mut opts: Peekable<I>) -> Resu
             std::process::exit(0);
         }
         if param_str.starts_with('-') {
-            return Err(format!("unrecognized option: {param:?}"));
+            return Err(format!("unrecognized option '{}'", param.to_string_lossy()));
         }
         if from.is_none() {
             from = Some(param);

src/params.rs

@@ -195,7 +195,7 @@ pub fn parse_params<I: Iterator<Item = OsString>>(mut opts: Peekable<I>) -> Resu
             Err(error) => return Err(error),
         }
         if param.to_string_lossy().starts_with('-') {
-            return Err(format!("unrecognized option: {param:?}"));
+            return Err(format!("unrecognized option '{}'", param.to_string_lossy()));
         }
         if from.is_none() {
             from = Some(param);

tests/integration.rs

@@ -39,9 +39,7 @@ mod common {
             cmd.assert()
                 .code(predicate::eq(2))
                 .failure()
-                .stderr(predicate::str::starts_with(
-                    "unrecognized option: \"--foobar\"",
-                ));
+                .stderr(predicate::str::contains("unrecognized option '--foobar'"));
         }
         Ok(())
     }