cmd/retrybuilds: add -branch flag
cmd/retrybuilds: add -branch flag

Works for detecting new flaky builds from different pages, but I
can't actually clear them yet. (caching bug in the dashboard?)

Change-Id: Iad9c3a5aef2016a81ad71559bb140044b68e42e2
Reviewed-on: https://go-review.googlesource.com/14732
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,proppy/build,golang/build,proppy/build,proppy/build,golang/build,proppy/build,golang/build