Pretty consistently (but not 100%) https://gerrit.wikimedia.org/r/#/c/233660/ is failing with:
```
13:36:50 DEBUG:zuul.Repo:Updating repository src/extensions/VisualEditor
13:36:50 Traceback (most recent call last):
13:36:50 File "/usr/bin/zuul-cloner", line 10, in <module>
13:36:50 sys.exit(main())
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/zuul/cmd/cloner.py", line 160, in main
13:36:50 cloner.main()
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/zuul/cmd/cloner.py", line 155, in main
13:36:50 cloner.execute()
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/zuul/lib/cloner.py", line 68, in execute
13:36:50 self.prepareRepo(project, dest)
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/zuul/lib/cloner.py", line 133, in prepareRepo
13:36:50 repo.update()
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/zuul/merger/merger.py", line 180, in update
13:36:50 origin.update()
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/git/remote.py", line 536, in update
13:36:50 self.repo.git.remote(scmd, self.name, **kwargs)
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/git/cmd.py", line 440, in <lambda>
13:36:50 return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/git/cmd.py", line 834, in _call_process
13:36:50 return self.execute(make_call(), **_kwargs)
13:36:50 File "/usr/share/python/zuul/local/lib/python2.7/site-packages/git/cmd.py", line 627, in execute
13:36:50 raise GitCommandError(command, status, stderr_value)
13:36:50 git.exc.GitCommandError: 'git remote update origin' returned with exit code 1
13:36:50 stderr: 'error: object file .git/objects/a3/3b082c13c0f154fc498e51977f380423c4572f is empty
13:36:50 error: object file .git/objects/a3/3b082c13c0f154fc498e51977f380423c4572f is empty
13:36:50 fatal: loose object a33b082c13c0f154fc498e51977f380423c4572f (stored in .git/objects/a3/3b082c13c0f154fc498e51977f380423c4572f) is corrupt
13:36:50 fatal: The remote end hung up unexpectedly
13:36:50 error: Could not fetch origin'
13:36:50 Build step 'Execute shell' marked build as failure
```
E.g. https://integration.wikimedia.org/ci/job/mwext-qunit/4397/console and https://integration.wikimedia.org/ci/job/mwext-qunit/4399/console .
Maybe the source repository it's cloning from is corrupt and needs to be re-cloned from the canonical one?