Track autospan progress: Minor fix.

First change:
-------------
The current cron-job is failing when executing:
  ```
  git checkout main
  ```
with the error:
  ```
  Already on 'main'
  ```
Ignore the return value to avoid the problem.

Second change:
-------------
By default, I suspect `subprocess.run` plumb the child stdout/stderr to
the terminal output, instead of its own stderr/stdout. This prevent me
from knowing about error when running the script in a cronjob where both
stderr/stdout are redirected to a file.

Bug: 361375804
Change-Id: I5f0b96f40ec85e0206f1b56d94a0002ca0500319
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5833389
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356942}