[milo] make Gerrit queries faster.
[milo] make Gerrit queries faster.

SKIP_MERGEABLE allows Gerrit to skip potentially expensive and slow
computation. This has already helped CQ reduce Gerrit latency 2-3x.

R=hinoka, nodir

Bug: 924611
Change-Id: If9daf065f1f35f74d1d772bef51db4d8dae97eb8
Reviewed-on: https://chromium-review.googlesource.com/c/1435946
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

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