Fix autorebase error reporting

Fix autorebase error reporting

The logic used for detecting the commit breaking an autorebase does not
work correctly if the offending commit is not the first one applied
during the "reverse rebase".  Fix by using REBASE_HEAD instead of
processing the output of "git status" in a convoluted way.

Furthermore, the approach used for identifying the first offending merge
request in the case of a successful autorebase followed by a failed
build only works correctly if the base branch is not autorebased itself.
Since a solution that would work correctly for a branch autorebased on
top of a branch that only moves forward does not work correctly for a
branch autorebased on top of another autorebased branch and vice versa,
accurately identifying the most likely culprit after a successful
autorebase is a very complicated and brittle task.  Since reporting no
details at all is arguably better than reporting false details, only
produce a minimal error notification if the build fails after a
successful autorebase.