fix(git): sort commits in topological order (#415)

Commit a1b4b5b ("fix(git): sort the commits in topological order"), changed
the order from `TIME` to `TIME | TOPOLOGICAL`. According to the docs, this
is equivalent to `git log --date-sort`:

* https://github.com/libgit2/libgit2/blob/v1.7.1/include/git2/revwalk.h#L33-L38

This breaks down in the following scenario:

1. A PR is open.
2. A release v1 is made.
3. A PR is merged.
4. A relase v2 is made.

The git history for this would be:

```
- v2 release
- pr merge commit
- v1 release
- actual pr commit
```

This directly spills into the changelog produced by `git-cliff`, misattributing
commits that were merged in v2 to v1 retroactively when v2 is made.

You can see this with `git log`. If you pass `--topo-order` there:

```
- v2 relase
- pr merge commit
- actual pr commit
- v1 relase
```

With this change we only pass `Sort::TOPOLOGICAL` in `git-cliff`, which
produces the very same results as `git log --topo-order`.

git-cliff-core/src/repo.rs

@@ -49,7 +49,7 @@ impl Repository {
 		exclude_path: Option<Vec<Pattern>>,
 	) -> Result<Vec<Commit>> {
 		let mut revwalk = self.inner.revwalk()?;
-		revwalk.set_sorting(Sort::TIME | Sort::TOPOLOGICAL)?;
+		revwalk.set_sorting(Sort::TOPOLOGICAL)?;
 		if let Some(range) = range {
 			revwalk.push_range(&range)?;
 		} else {