Draft: atlas_git_merge: warn about sweep:ignore MRs before merge

Move the "sweep:ignore" warning into into its own pre-merge script. This will allow the release coordinator to check for potential conflicts before they are being resolved.

@tsulaia feel free to test this. To do so you will need a separate checkout of this branch and then run the script from your actual athena repository that you use for the merge, e.g.:

$ path/to/athena> ../athena2/Build/AtlasBuildScripts/atlas_git_merge.sh 24.0

resolved all threads

Replaced by !78834 (merged).

closed