Merger: log non Git exceptions on merge

Some git merge commands fails for me with an AssertionError coming from
GitPython:

 AssertionError: len([
    "5a0a437ee9a6fd4b30484dfdc2455c83ea00a058\t\t'refs/changes/80/111180/1'
of ssh://gerrit.wikimedia.org:29418/mediawiki/core\n"]
   )
   != len([
     'Total 10 (delta 9), reused 10 (delta 9)',
     ' * branch refs/changes/80/111180/1 -> FETCH_HEAD'
   ])

Zuul.Merger explicitly log merge exceptions at debug level, though we
should only load GitCommandError exceptions and properly log other
exceptions (such as AssertionError).

Change-Id: Idca682c0920b9e6eac3f12cc05634dfe304a0305
Merge "Merger: log non Git exceptions on merge"