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

lib/BUILD

diff --git a/lib/BUILD b/lib/BUILD
index 282643d..0f9620e 100644
--- a/lib/BUILD
+++ b/lib/BUILD
@@ -42,3 +42,45 @@
         "@slf4j-simple//jar",
     ],
 )
+
+java_library(
+    name = "jgit",
+    visibility = ["//visibility:public"],
+    exports = ["@jgit//org.eclipse.jgit:jgit"],
+    runtime_deps = [
+        ":slf4j-api",
+        "@javaewah//jar",
+    ],
+)
+
+java_library(
+    name = "jgit-archive",
+    visibility = ["//visibility:public"],
+    exports = ["@jgit//org.eclipse.jgit.archive:jgit-archive"],
+    runtime_deps = [":jgit"],
+)
+
+java_library(
+    name = "jgit-junit",
+    testonly = True,
+    visibility = ["//visibility:public"],
+    exports = ["@jgit//org.eclipse.jgit.junit:junit"],
+    runtime_deps = [":jgit"],
+)
+
+java_library(
+    name = "jgit-servlet",
+    visibility = ["//visibility:public"],
+    exports = ["@jgit//org.eclipse.jgit.http.server:jgit-servlet"],
+    runtime_deps = [":jgit"],
+)
+
+java_library(
+    name = "tukaani-xz",
+    exports = ["@tukaani-xz//jar"],
+)
+
+java_library(
+    name = "commons-compress",
+    exports = ["@commons-compress//jar"],
+)