Linkage check script assumes you are on a branch

[damccorm]

A common use case for linkage check is verifying someone else's PR, which does not involve any branches, but merging two non-branch references. The script assumes you are on a branch, so it does not work well for this use case.

Imported from Jira BEAM-11261. Original Jira may contain additional context.
Reported by: kenn.

[mohamedawnallah]

.take-issue

[mohamedawnallah]

Freeing up my assignment to this good-first-issue so other contributors can jump in if interested! If no one picks it up by then, I'm happy to submit a PR to close it out

[mohamedawnallah]

.free-issue

[zhan7236]

.take-issue