RepoPick : Add support for git pull

Allow user to git pull patchsets and dependencies.

Change-Id: If5520b45fe79836eac628b3caf0526f11f8953d9
(cherry picked from commit df646304bdcef329e3fe7c12b68107de1f4cd42a)
(cherry picked from commit 4dbb51c00338641acf00eb881fea293d81890724)