Buck: Fix plugin build by not merging manifests from dependent JARs

After posting this mail to mailing list [1], filing this issue [2],
and creating this pull request [3], Buck team provided a solution to
the problem: when multiple JARs contain MANIFEST files, the default
behaviour of java_binary() was to merge the content of those manifest
files together. That breaks our plugins, as the content from other JARs
interfere with the manifest content of plugins.

Recent Buck version [4] provides undocumented parameter to change this
behaviour.

[1] https://groups.google.com/forum/#!topic/buck-build/M7Nu71okdYY
[2] https://github.com/facebook/buck/issues/86
[3] https://github.com/facebook/buck/pull/87
[4] https://github.com/facebook/buck/commit/\
96ba1967ef941558bc4c12ce88d5eaeba0b23275

Change-Id: Ia228fb51cfe8db678c2551b9cd1802bdaf4d44fc

tools/default.defs

@@ -134,9 +134,9 @@ def gerrit_plugin(
   java_binary(
     name = name,
     manifest_file = ':%s__manifest' % name,
+    merge_manifests = False,
     deps = [
       ':%s__plugin' % name,
-      ':%s__manifest' % name,
     ] + static_jars,
     visibility = visibility,
   )