cmd/coordinator: add main revision commit date to bench upload
cmd/coordinator: add main revision commit date to bench upload

The downstream benchmarking tools need commit date to establish a basic
ordering (in the future they can rely on gitiles APIs or something, but
this is a quicker and faster way to an MVP).

Change-Id: Ic52f1346d08661e9e4a2ea81ef1721c275ad288f
Reviewed-on: https://go-review.googlesource.com/c/build/+/393641
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

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