Bazel: add support for unsigning jar files

One test is still failing:

  HttpPushForReviewIT#pushForMasterWithTopicOption()

Change-Id: I257f443557a998f0c2190bd07f01547368aeb1e1

lib/jgit/org.eclipse.jgit.junit/BUILD

@@ -1,6 +1,14 @@
+load('//tools/bzl:unsign.bzl', 'unsign_jars')
+
 java_library(
-  name = 'junit',
+  name = 'junit-signed',
   exports = ['@jgit_junit//jar'],
   runtime_deps = ['//lib/jgit/org.eclipse.jgit:jgit'],
   visibility = ['//visibility:public'],
 )
+
+unsign_jars(
+  name = 'junit',
+  deps = [':junit-signed'],
+    visibility = ['//visibility:public'],
+)