GitResource schema, add missing properties

[kdvolder]

Related to #668

In that issue's fix added support for fetch_tags property. However, I think there are other missing properties (the editor schema has not been updated in quite a while).

We should:

• look at all the properties in the documentation for git resource: https://github.com/concourse/git-resource
• determine which of them are missing from the editor schema
• add the properties to the schema and hover support and add corresponding test cases.

Note that properties may be missing from either the git resource Source schema but also from the corresponding get and put step schemas.

[kdvolder]

Missing from 'GitSource':

• private_key_user DONE
• private_key_passphrase DONE
• forward_agent DONE
• tag_regex DONE
• submodule_credentials DONE
• git_crypt_key DONE
• https_tunnel DONE
• commit_filter DONE
• version_depth DONE

[kdvolder]

Missing from GitGetParams schema:

• fetch_tags DONE
• short_ref_format DONE
• timestamp_format DONE
• describe_ref_options DONE

[kdvolder]

Missing from GitPutParams:

• returning DONE
• branch DONE

[kdvolder]

Fixes pushed.