Add option to ignore files from being committed

[MarcoIeni]

Motivations

The conversation started here.

Context: Release-plz runs git add . when running the release-pr command.
So before running, it checks if the git repository is clean.

We might want to ignore some files for some reasons:

• these files are produced by the CI and we don't want them to be committed in the release-pr.
• users might not want to commit some files changed by release-plz (like changelogs, Cargo.lock or Cargo.toml files) of some crate. Even though for this use-case we might have specific configurations for each crate.

Solution

• When checking if the repository is clean, ignore these files.
• After running git add ., remove these files from the git staging area.
• When release-plz detects that the repository is not clean, suggest to add these files to the ignored files list.

[MarcoIeni]

Instead of adding a custom flag, we could run cargo package --list and rely on cargo to only consider the relevant files. Docs here: https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields

When running the diff algorithm, we should only consider these files.

[MarcoIeni]

Close in favor of #721