Avoid git commands when running against a fork

[artem-shelkovnikov]

See failure of CI in PR #3090: https://buildkite.com/elastic/connectors/builds/15589#01945040-19d0-4239-915a-173479f8aee8

++ git switch -
--
  | Warning: you are leaving 2 commits behind, not connected to
  | any of your branches:
  |  
  | 824f57b6 use rstrip for lists to remove whitespace
  | cf502ec0 remove '' as a space with trailing comma
  |  
  | If you want to keep them by creating a new branch, this may be a good time
  | to do so with:
  |  
  | git branch <new-branch-name> 824f57b6
  |  
  | Switched to branch 'main'
  | Your branch is up to date with 'origin/main'.
  | ++ git checkout meghanmurphy1:confluence-trailing-comma
  | error: pathspec 'meghanmurphy1:confluence-trailing-comma' did not match any file(s) known to git
  | 🚨 Error: The command exited with status 1
  | user command error: exit status 1

I happens because forks are not known to Buildkite agent. Forks will not by default give permissions to Elasitc agents, so we cannot really push anything into them. This PR just makes forks treated same way we treat main branch: just report failures, don't try to fix them.

[artem-shelkovnikov]

buildkite test this

[artem-shelkovnikov]

buildkite test this

[jedrazb]

🚢

[erikcurrin-elastic]

1 is false?

[artem-shelkovnikov]

Fun unix stuff :D Functions actually return exit codes, not regular variables:

https://stackoverflow.com/questions/17336915/return-value-in-a-bash-function

So 0 = "all good", non-0 are "something is wrong"

[github-actions]

💔 Failed to create backport PR(s)

The backport operation could not be completed due to the following error:
There are no branches to backport to. Aborting.

The backport PRs will be merged automatically after passing CI.

To backport manually run:
backport --pr 3095 --autoMerge --autoMergeMethod squash