[SalesForce] How to retrieve Lightning Web Components using Ant Migration tool or mdapi:retrieve command from SFDX

One possible solution is to use Packages as a container for your code. This allows you to retrieve only the code in your package from an org which ignores all other metadata in the org such as managed package fields, standard Salesforce fields, and any customer created fields.

To test it out, go to Setup->Create->Packages in your source org. Create an unmanaged package and use the Add Components button to add all the metadata you want to bundle in your package. Then, retrieve all metadata from the package into a folder. You'll wind up with a folder containing all the metadata and a package.xml file formatted to allow deployment into a Package in your target orgs.

The key difference in the package.xml is the inclusion of the element which flips on the deploy to package functionality.

The one catch is the target org will need to have a package with the same name already created before the deployment but this is a one time manual step per target org.

I just wrote up the build.xml targets in a different answer you can reference here: https://salesforce.stackexchange.com/a/31437/4832

There are, however, some drawbacks. Some metadata types won't deploy this way. For example, ActionOverrides don't work this way as they can't be bundled in a package. As a workaround, you can create a normal bundle of metadata for use only in deploying the types you can't deploy as part of a package.

I think it might be a problem with the location of your ant-salesforce.jar file. I always include it in my repos in a lib folder. Take a look at my example repo for a structure that works.

Then, your command would include the -lib switch

ant -lib lib/ant-salesforce.jar deployunpackaged

Note, another way to deploy from Github is to use Andy Fawcett's fancy button.