Fix Truth dependencies

The used version of Truth indirectly depends on diffutils. Add it
similarly as we did for Gerrit core (see I80527c9ea4). Gerrit core also
specifies Guava as runtime dependence, so do it here too.

Change-Id: I3d106f3b990fa4447ac772cad4e2c4b3c2b50811

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 47e3966..3a07abe 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -116,6 +116,13 @@
     sha1 = "b5768f644b114e6cf5c3962c2ebcb072f788dcbb",
 )
 
+# Indirect dependency of truth
+maven_jar(
+    name = "diffutils",
+    artifact = "com.googlecode.java-diff-utils:diffutils:1.3.0",
+    sha1 = "7e060dd5b19431e6d198e91ff670644372f60fbd",
+)
+
 maven_jar(
     name = "soy",
     artifact = "com.google.template:soy:2018-03-14",