Fix MetadataEntry's equals and hashCode methods

src/main/java/net/fabricmc/mappingio/tree/MemoryMappingTree.java

@@ -1714,6 +1714,8 @@ static final class MetadataEntry {
 
 		@Override
 		public boolean equals(Object other) {
+			if (other == this) return true;
+
 			if (!(other instanceof MetadataEntry)) {
 				return false;
 			}
@@ -1723,6 +1725,11 @@ public boolean equals(Object other) {
 			return this.key.equals(entry.key) && this.value.equals(entry.value);
 		}
 
+		@Override
+		public int hashCode() {
+			return key.hashCode() | value.hashCode();
+		}
+
 		final String key;
 		final String value;
 		final boolean overrideExisting;