Sporadic Mellanox Open MPI CI failures with the error "fatal: reference is not a tree"

[artemry-nv]

Mellanox Open MPI CI may fail from time to time during git checkout phase with the error fatal: reference is not a tree.

Workaround: re-run Mellanox CI with /azp run PR comment (for details see README).

Potential root cause: Azure Pipelines issue (not yet fixed)

Initial discussion: #7134

[artemry-nv]

It looks like the related Azure Pipelines issue has been fixed.
I'm monitoring whether such error appears again.