Don't double-include if global-requirements

global-requirements is a fully self-contained file, containing both
test and normal requirements. In the situation where the target repo
has a global-requirements.txt file, it should be treated as the only
file that matters- otherwise the mirror builder will hit double include
issues.

Change-Id: I01fd1e0d7f0923456f35a0f931159850cb52e3e7