[petsc-dev] CI update: please rebase before starting test pipelines

[Balay, Satish]

PETSc devs using CI at gitlab,

With the current CI changes - before starting a test pipeline on a
branch at gitlab, its best to rebase the branch over current master as
of now [or maint - if appropriate].

If a test pipeline is created with non-rebased branch, some of the
stage-2 jobs will be stuck - so the pipeline won't complete.

Satish