[autoroll] 'git fetch' the child repo as well as the parent
[autoroll] 'git fetch' the child repo as well as the parent

gclient no longer does this automatically.

Bug: chromium:938627
Change-Id: Ia6549e922fa295d66dbe82b2cef87c2984f530a9
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/198841
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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