Don't commit excluded files

[MarcoIeni]

Motivations

We should honor excluded and included Cargo manifest files. Docs here.

You can list the files to include in the package with cargo package --list

Solution

• When running the diff algorithm, we should only consider the files in cargo package --list.
• When checking if the repository is clean, ignore files not in cargo package --list.
• git add only files in cargo package --list
• When release-plz detects that the repository is not clean, suggest adding these files to the ignored files list in Cargo.toml. Link to the docs.

[MarcoIeni]

This feature is handy if you don't want release-plz to open a PR for changes in every file. E.g. if you don't want release-plz to raise a PR for a change in the README.md file, you could add README.md file to the excluded files.
Same for CHANGELOG.md.

[lutaok]

Hi!
I will gladly take on this issue!

[MarcoIeni]

First item in the list completed by #1089 🎉
This was the most important bit, but it would be nice to implement the other ones, too 👍

[zvolin]

Hey, I was wrong in my mention, our issue turned out to be the file in tree which was also matched by .gitignore rules.