fix: Ignore unknown keys in input

This seems to be an input validation that even I forgot about, as in
README it says 'unknown fields are ignored' [1].

For now simply ignoring such fields should be enough to tackle the new
wwwauth[] key in Git 2.41.0 [2]. In the future if people complain that
these lines are not echoed back, we can consider adding a HashMap<String,
Vec<String>> field.

[1] https://github.com/Frederick888/git-credential-keepassxc/blob/5cb8047d6d9684d5c63081cc675fff850f6e221d/README.md?plain=1#L143
[2] git/git@af5388d...5f2117b

src/git.rs

@@ -85,10 +85,7 @@ macro_rules! message_from_to_string {
                                 msg.$field_name = Some(pair[split_at + 1..].to_owned());
                             },
                         )*
-                            _ => return Err(GitMessageParsingError {
-                                message: format!("Unknown key {}", key),
-                                source: s.to_owned(),
-                            }),
+                            _ => {},
                     }
                 }
                 Ok(msg)
@@ -198,4 +195,16 @@ mod tests {
         message.set_string_fields(&advanced_fields);
         assert_eq!(string1 + &string2 + "\n", message.to_string());
     }
+
+    #[test]
+    fn test_03_ignore_unknown_fields() {
+        let string = "url=http://example.com\nfoo=bar\n".to_owned();
+        let message = GitCredentialMessage::from_str(string.as_str()).unwrap();
+        assert!(message.url.is_some());
+        assert_eq!(message.url.as_ref().unwrap().as_str(), "http://example.com");
+        assert_eq!(
+            "url=http://example.com\n\n".to_string(),
+            message.to_string()
+        );
+    }
 }