Expose isShallow from ScmProviderRepository

### New feature, improvement proposal

Git supports [shallow clones](https://git-scm.com/docs/shallow). However those behave differently in certain aspects (e.g. #1378). Therefore a getter method exposing for a repository if it is shallow cloned or not would be beneficial.

Start by locating ScmProviderRepository and reading how repository state, including shallow clones, is represented. Add an accessor that reports whether the repository is shallow-cloned, then verify the exposed value against the repository state; the issue does not name a specific test.

Written by the indexing model from the issue text.