codereview.settings: add GIT_NUMBER_FOOTER setting.
codereview.settings: add GIT_NUMBER_FOOTER setting.

This will be used in repositories that use Cr-Commit-Position footers
to generate them at the time of git cl land instead of gnumbd.

R=iannucci@chromium.org
BUG=642493

Review-Url: https://codereview.chromium.org/2324583002

primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools