Make script/uplift.py idempotent

[wknapik]

Discussion thread https://bravesoftware.slack.com/archives/CHGKGMHDJ/p1715601103401679?thread_ts=1715437662.047869&cid=CHGKGMHDJ

If the script fails after creating a branch, subsequent runs will also fail.
This can happen as a result of #36336, or due to any other issues.
So far, there's been 100 such failures reported in to the uplift channel.

cc @bsclifton