Bug 1058036 - Ensure dependency isn't lost when ORing InstallManifest; r=glandium

The install manifest processor starts with an empty InstallManifest and
uses |= to "concatenate" instances. It became pretty obvious when
developing some patches that add more preprocessed files to install
manifests that the source install manifest dependency was getting
lost during the |= operation. This patch fixes it.

The solution is not ideal performance wise. But slightly worse
performance (only after config.status, however) is better than
clobbers.

A test has been added to ensure this doesn't regress.

--HG--
extra : rebase_source : 848aebbbc935ce2bca2d3fcc85d1df534e734e0d