Not doing so causes a failure in the 'git checkout $base_rev' step as visible here: https://build.chromium.org/p/client.skia.fyi/builders/Infra-PerCommit-Trybot/builds/4995
This causes the checkout to remain in the "rebase in progress" state.

BUG=chromium:645955
BUG=skia:5749

Committed: https://chromium.googlesource.com/chromium/tools/depot_tools/+/9736fe8938e67337bb45dccf79ae8699b9625fa6
Review-Url: https://codereview.chromium.org/2328313002