Add git.depth key to override shallow clone behavior

By default Antora shallow clones the repo to build the docs. In some cases the project version information is coming from git (release tags). Because of shallow cloning, the history, as well as the previous tags are not available so the build process is not able to find the latest release tag (to know what is the current version).

With a git.depth key, this could have been fixed, by default the value would be 1 (current behavior), 0 would mean cloning the full history and other values would specify the number of commits in the history.