Split off library classes from server/patch/ into diff/

The server library is large, and its resulting compilation time gets
in the way of edit/compile/debug cycles.

The new diff library files depend mainly on JGit, and are independent
of the Guice stack in the server library.

Change-Id: I483992fff91873b1c1751c16fe3a83b823f4f8fa

java/com/google/gerrit/server/BUILD

@@ -38,6 +38,7 @@ java_library(
         "//java/com/google/gerrit/metrics",
         "//java/com/google/gerrit/prettify:server",
         "//java/com/google/gerrit/reviewdb:server",
+        "//java/com/google/gerrit/server/diff",
         "//java/com/google/gerrit/server/ioutil",
         "//java/com/google/gerrit/util/cli",
         "//java/com/google/gerrit/util/ssl",