multirun: propagate the entire runfiles of dependencies

[kevingessner]

runfiles can include more than just transitive_files, such as empty_filenames. The best way to ensure all parts of runfiles are propagated is to use merge so Bazel takes care of every property.

This fixes commands that call out to py_binary, for instance, which relies on empty_filenames.

[ash2k]

This stuff is so poorly documented... Thanks for fixing it! So with this fix it works for some use case that you have?

Can you sign the CLA please? https://github.com/atlassian/bazel-tools#contributing

[kevingessner]

Yeah, this fixes a case where command points to a py_binary, which uses the empty_filenames list to create __init__.py files (https://docs.bazel.build/versions/master/be/python.html#py_binary.legacy_create_init). If those runfiles aren't propagated, the command won't work right.

CLA signed!

[kevingessner]

thank you!