git cl: remove code related to pending refs and gnumbd.

Refactor _GitNumbererState to two functions and remove no longer
useful tests.

BUG=644915
R=agable@chromium.org

Change-Id: If5e3e3b141aee192211f6af130b01f20f9afdbfe
Reviewed-on: https://chromium-review.googlesource.com/431976Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>