Allow repopick to cherry-pick a specific patch set

Use 'repopick 123456/9' where '123456' is the change number
and '9' is the desired patchset.

Change-Id: I2d9f6939fbde50b2a6057b75d2e7f722be5a3e21
(cherry picked from commit 73b697d956fc22fe9ae80adb64a1661a04f90903)