GitLab

Add tip about @{u} as short-hand for the upstream branch when trying
to squash commits using `git rebase -i @{u}`.

Bug: None
Change-Id: I8b06ccdb976fdfe84eac858d3871fac8f85c2f74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993498Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Toby Huang <tobyhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729862}