Fix git_footers to accept refs with periods in them.
Fix git_footers to accept refs with periods in them.

For example, v8 has refs/heads/1.4.2

TBR=machenbach@chromium.org
BUG=chromium:674448

Change-Id: Ia3405ee402a01153fc9744457d8fee9385446cae
Reviewed-on: https://chromium-review.googlesource.com/420285
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools