[CI] Fail more informatively if git merge-base fails
[CI] Fail more informatively if git merge-base fails

We've seen a couple of failed CI jobs recently, which appear to be
going wrong here. Not sure what's going wrong, but this might give a
bit more information about what's happening.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan