Quell warnings by removing unused Struct fields

[blairconrad]

When git-absorb is built, we're beset by warnings:

warning: field `trailing_newline` is never read
  --> src/owned.rs:54:9
   |
51 | pub struct Block {
   |            ----- field in this struct
...
54 |     pub trailing_newline: bool,
   |         ^^^^^^^^^^^^^^^^
   |
   = note: `Block` has derived impls for the traits `Clone` and `Debug`, but these are intentionally ignored during dead code analysis
   = note: `#[warn(dead_code)]` on by default

warning: fields `old_id` and `new_id` are never read
   --> src/owned.rs:209:9
    |
207 | pub struct Patch {
    |            ----- fields in this struct
208 |     pub old_path: Vec<u8>,
209 |     pub old_id: git2::Oid,
    |         ^^^^^^
210 |     pub new_path: Vec<u8>,
211 |     pub new_id: git2::Oid,
    |         ^^^^^^
    |
    = note: `Patch` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis

warning: `git-absorb` (lib) generated 2 warnings

Removing a few unused fields quiets them right up.