Factor out JGit operations into a Helper

Separate concerns by factoring out all the JGit machinery
from the core owners logic.
This is a prerequisite for unit test and mocking external
layers.

Change-Id: Ic7bd5101d178e5fea615040eb5bf7c04413628bb
diff --git a/README.md b/README.md
index 24934c5..5e5b965 100644
--- a/README.md
+++ b/README.md
@@ -64,3 +64,4 @@
 NOTE: the owners-common folder is producing shared artifacts for the two plugins
 and does not need to be built separately being a direct dependency of the build
 process. Its resulting .jar must not be installed in gerrit plugins directory.
+