[milo] Start caching git.GetHistory responses.
[milo] Start caching git.GetHistory responses.

Also add a metric for GetHistory so we can have more information to
diagnose.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, 9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: 793494
Change-Id: Id0072e5cdc834ca2faf52fc10305822adf14d270
Reviewed-on: https://chromium-review.googlesource.com/834797
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go