[gold] Fix index error when exactly one new commit
[gold] Fix index error when exactly one new commit

In https://skia-review.googlesource.com/c/buildbot/+/224521 the logic
was changed to only have one commit instead of two or more.
This branch wasn't also modified, occasionally causing crashes.

Change-Id: I1fdb7ff4fd00424bb9b22b7d27aae4276d14acf8
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/232336
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot