tree: Add a test showing failure to roundtrip

We can see it even in the existing test fixtures:
As the types are currently defined, some trees have more than one valid
git representation, meaning that with these types, we cannot roundtrip
from git representation to gix-object types and back to git
representation.

The issue comes from storing the `EntryMode` as an integer.

In git, the source of truth is the string that represents the mode in
octal form, represented as up to 6 characters.
For an `EntryMode` representing a Tree, there are two valid ways to
represent them in `git`: `"40000"` and `"040000"`.
We see both representations in the wild.

Because `"40000"` and `"040000"` become identical once parsed as `u16`,
there is no possible way to ensure roundtrip without enriching the data
representation with at least one more bit of information to indicate
whether the mode had leading zeroes.

This is not an issue for `Blob`, `BlobExecutable`, `Link` or `Commit`
because each of them starts with a 1 when expressed as 6 digits.

Author: pierrechevalier83

gix-object/tests/object/tree/from_bytes.rs

@@ -101,6 +101,10 @@ fn special_trees() -> crate::Result {
             expected_entry_count,
             "{name}"
         );
+        // Show we can roundtrip
+        let mut buf: Vec<u8> = Default::default();
+        actual.write_to(&mut buf).expect("Failed to write bytes to buffer");
+        assert_eq!(buf, fixture);
     }
     Ok(())
 }