If a ref is incorrectly set, it will affect later runs.
For example, [1] caused a bad refspec to be added to the fetch config,
(+refs/branch-heads/refs/branch-heads/6.7:refs/branch-heads/refs/branch-heads/6.7)
which affects all subsequent runs in the same bot: [2], [3].

This makes bot_update clear the fetch config before each run, so it is more
robust against these kind of failures.

[1] https://logs.chromium.org/v/?s=v8%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8942033142534034848%2F%2B%2Fsteps%2Fbot_update%2F0%2Fstdout
[2] https://logs.chromium.org/v/?s=v8%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8941307871443322944%2F%2B%2Fsteps%2Fbot_update%2F0%2Fstdout
[3] https://logs.chromium.org/v/?s=v8%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8941333453153279680%2F%2B%2Fsteps%2Fbot_update%2F0%2Fstdout

Bug: 862547
Change-Id: I2f849c604656e81ebd7377465d287226b8bdea1a
Reviewed-on: https://chromium-review.googlesource.com/1135645
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>