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

.gitmodules

diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..b339fa4
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "modules/jgit"]
+	path = modules/jgit
+	url = ../jgit