Link to JGit source as git submodule

Using a submodule to bring in the source files vastly simplifies
updating JGit. Gitiles developers can bump the SHA-1 of the submodule
and not deal with the Maven rebuild process for JGit.

This follows the approach taken in Gerrit in I16370e57f27.

Change-Id: Ib35f7bad8b8f61e73f416937a387e3675bf1e3a2

tools/workspace_status.py

diff --git a/tools/workspace_status.py b/tools/workspace_status.py
index aae9318..bd0689a 100644
--- a/tools/workspace_status.py
+++ b/tools/workspace_status.py
@@ -36,3 +36,11 @@
 
 
 print("STABLE_BUILD_GITILES_LABEL %s" % revision(ROOT, ROOT))
+for kind in ['modules']:
+    kind_dir = os.path.join(ROOT, kind)
+    for d in os.listdir(kind_dir):
+        p = os.path.join(kind_dir, d)
+        if os.path.isdir(p):
+            v = revision(p, ROOT)
+            print('STABLE_BUILD_%s_LABEL %s' % (os.path.basename(p).upper(),
+                                                v if v else 'unknown'))