Cargo: Support ignoring git dependencies

An example of git dependency:
```toml
[dependencies]
regex = { git = "https://github.com/rust-lang/regex" }
```
Currently, there is no way to tell dependabot to ignore such a dependency update in `Cargo.lock`, but to specify its name.
Ignoring syntax could be
```yml
update-types: ["git"]
```
https://github.com/dependabot/dependabot-core/blob/main/common/lib/dependabot/config/ignore_condition.rb

Unless this feature is already supported, but not documented yet.

Start with common/lib/dependabot/config/ignore_condition.rb and inspect how ignore conditions and update-types are currently parsed, then check whether Cargo git dependencies are handled elsewhere. Confirm whether the proposed "git" condition already works; done means the behavior is supported or its documented limitation and required configuration are clear.

Written by the indexing model from the issue text.