java_binary: Don't merge manifest_file with MANIFEST.MF files from dependent JARs

Created by: davido

Issue [1] fixed the problem that the content from manifest_file was overridden. However, the content is still merged from all MANIFEST.MF from dependent JARs. So the result is totally messed up.

If manifest_file is provided as input to java_binary, there is no point of merging anything. The content of input manifest_file shouldn't change at all.

[1] https://github.com/facebook/buck/issues/44