Cancelled error on invalid index entries

`crates-index-diff` now yields `DiffForEach(Diff(Cancelled))` error on seemingly-okay git history (version 12 worked fine):

```console
$ git init repo
$ mkdir -p repo/aw/s-
$ echo '{"name":"aws-foo","vers":"0.0.1","deps":[],"cksum":"","features":{},"yanked":false}' > repo/aw/s-/aws-foo
$ git -C repo add .
$ git -C repo commit -m 'commit'
# note down the commit hash here
$ echo '{"name":"aws-foo","vers":"0.0.2","deps":[],"cksum":"","features":{},"yanked":false}' >> repo/aw/s-/aws-foo
$ git -C repo add .
$ git -C repo commit -m 'commit'
# note down this commit hash too
$ git -C repo remote add origin origin
$ git -C repo diff ..
diff --git a/aw/s-/aws-foo b/aw/s-/aws-foo
index f512d82..9bff708 100644
--- a/aw/s-/aws-foo
+++ b/aw/s-/aws-foo
@@ -1 +1,2 @@
{"name":"aws-foo","vers":"0.0.1","deps":[],"cksum":"","features":{},"yanked":false}
+{"name":"aws-foo","vers":"0.0.2","deps":[],"cksum":"","features":{},"yanked":false}
$ cargo new --bin whatever && cd whatever
$ echo 'crates-index-diff = "15"' >> Cargo.toml
$ cat src/main.rs
use crates_index_diff::{git, index::CloneOptions, Index};
use std::sync::atomic::AtomicBool;

fn main() {
let repo = git::open("../repo").unwrap();
let origin_url = repo
.find_remote("origin")
.unwrap()
.url(git::remote::Direction::Fetch)
.unwrap()
.to_bstring()
.to_string();
let index = Index::from_path_or_cloned_with_options(
"../repo",
git::progress::Discard,
&AtomicBool::default(),
CloneOptions {
url: origin_url,
..Default::default()
},
)
.unwrap();

index.changes("", "").unwrap();
}
$ cargo r
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: DiffForEach(Diff(Cancelled))', src/main.rs:24:41
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The `origin_url` bit is to ensure we always use the existing path rather than ever try to clone. At least that was the intent, and that worked back in version 12. Don't know if it's relevant here.

Start with the reproducer in src/main.rs, especially Index::from_path_or_cloned_with_options and index.changes, and compare the behavior with crates-index-diff version 12. Recreate the two-commit invalid-index history and trace why the diff returns DiffForEach(Diff(Cancelled)). Done means the supplied history can be processed without that error, with regression coverage for the case.

Written by the indexing model from the issue text.