Exclude expectrl and imara-diff from grouped version updates

Dependabot is working again for cargo dependencies: #2245 was due
to dependabot/dependabot-core#13345,
fixed in dependabot/dependabot-core#13359.

However, now we get an analogous problem in here in `gitoxide` with
`expectrl` and `imara-diff` to the problem previously encountered
in `cargo-smart-release` with `pulldown-cmark`. That was seen in
GitoxideLabs/cargo-smart-release#85, where
even though it is held back in `dependabot.yml`, Dependabot
proposed updates to it in `Cargo.toml`. The analogous problem here
can be seen in #2268, which includes `expectrl` and `imara-diff`.

In GitoxideLabs/cargo-smart-release#86, we
worked around the problem in `cargo-smart-release` by explicitly
excluding `pulldown-cmark` from grouped updates, so that PRs would
be made without it even of Dependabot were to wrongly detect that
it should attempt to upgrade it. That was even more effective than
anticipated, in that Dependabot also refrained from opening extra
non-grouped PRs for it (it heeded the version restrictions again).

This attempts an analogous change here in `dependabot.yml` for
`gitoxide`, excluding `expectrl` and `imara-diff` from grouped
version updates. Hopefully this will at least allow Dependabot
grouped version update PRs to be made that don't bump those crates.

Author: EliahKagan

.github/dependabot.yml

@@ -30,6 +30,9 @@ updates:
     groups:
       cargo:
         patterns: ['*']
+        exclude-patterns:
+          - expectrl
+          - imara-diff
 
   - package-ecosystem: github-actions
     directory: '/'