Consume JGit from development tree: Add missing dependency

To run the tests, hamcrest_library is needed for jgit project and
because of this issue: [1], bazel tries to resolve this external
dependency in gerrit project. Add hamcrest library to WORKSPACE file.

Test Plan:

* switch to using JGit from the development tree
* run the tests:

  bazel test --define jgit-dev=1 //...

[1] https://github.com/bazelbuild/bazel/issues/2391
Change-Id: I0fd16c26a6048aa1c852cf097cfca4ab230d0496
diff --git a/WORKSPACE b/WORKSPACE
index 7ccb94a..eb27437 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -713,6 +713,13 @@
     sha1 = "42a25dc3219429f0e5d060061f71acb49bf010a0",
 )
 
+# Only needed when jgit is built from the development tree
+maven_jar(
+    name = "hamcrest_library",
+    artifact = "org.hamcrest:hamcrest-library:1.3",
+    sha1 = "4785a3c21320980282f9f33d0d1264a69040538f",
+)
+
 TRUTH_VERS = "0.32"
 
 maven_jar(