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 <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>

cmd/coordinator/buildstatus.go

@@ -1388,6 +1388,7 @@ func (st *buildStatus) uploadBenchResults(baseline string) (err error) {
 	// Prepend some useful metadata.
 	var b strings.Builder
 	fmt.Fprintf(&b, "experiment-commit: %s\n", st.Rev)
+	fmt.Fprintf(&b, "experiment-commit-time: %s\n", st.revCommitTime)
 	fmt.Fprintf(&b, "baseline-commit: %s\n", baseline)
 	fmt.Fprintf(&b, "benchmarks-commit: %s\n", st.SubRev)
 	fmt.Fprintf(&b, "post-submit: %t\n", st.trySet == nil)