[scheduler] do not confuse reader of debug logs.
[scheduler] do not confuse reader of debug logs.

w/o this CL, debug log reads like this:
  [01:53:59.324] Repo: https://x/y/z, Refs: refs/heads/master
  [01:53:59.371] Ref refs/heads/master updated: deadbeef => beefcafe
  [01:53:59.417] Ref refs/heads/master: log old..new is not found, investigating further...
  [01:53:59.495] Emitting a trigger https://x/y/z/+/refs/heads/master@beefcafe

R=vadimsh

Bug: 853982
Change-Id: I5eee65e736c5de64d9ed22422963d9460270189a
Reviewed-on: https://chromium-review.googlesource.com/1107309
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@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