fix: explicitly fetch base branch before rebase in sync

[javoire]

Fixes an issue where stack sync wouldn't update branches to the latest base branch (e.g., master) in certain configurations.

Problem:
When rebasing onto a base branch like origin/master, the sync relied on the general git fetch origin to update the tracking ref. However, in repos with limited refspecs or certain git configurations, this fetch may not reliably update origin/master.

Solution:
Explicitly fetch the base branch using FetchBranch() right before rebasing onto it. This ensures the tracking ref is always up-to-date before the rebase.

The fetch is non-fatal, so if it fails (e.g., network issues), the sync continues with the existing ref.