analysis/app: add "commit" tooltip to trend
analysis/app: add "commit" tooltip to trend

This makes the graphs produced by /trend much more useful by exposing
the actual commit SHA in addition to the index.

Change-Id: Ia8b11278752bcd95322740afb5c17783db74e944
Reviewed-on: https://go-review.googlesource.com/47510
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/perf,golang/perf