servo-mac7 can't complete git checkout

[jdm]

We've seen this consistently in servo/servo#17042:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1445, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1299, in _inlineCallbacks
    result = g.send(result)
  File "/usr/local/lib/python2.7/dist-packages/buildbot/process/buildstep.py", line 322, in startStep
    result = yield self.run()
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1445, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
--- <exception caught here> ---
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1299, in _inlineCallbacks
    result = g.send(result)
  File "/home/servo/buildbot/master/factories.py", line 40, in run
    got_rev, rev
exceptions.Exception: Actual commit (8daa4ef070f0996b482a96e04bc86ecc21f7efcd) differs from requested commit (1a719a103ea452ba1a1645b0bea93cb8dfcf16e9)

[nox]

@jdm This happens when we stop builds by hand in Buildbot, lock files don't get cleaned.

[nox]

@jdm servo-mac4 too has the issue, for the mac-rel-wpt2 job.

[nox]

@jdm See this error, during the Git step:

fatal: Unable to create '/Users/servo/buildbot/slave/mac-rel-wpt1/build/.git/index.lock': File exists.

[larsbergstrom]

This should be fixed now. I basically logged into all of them and did a:

find . -name index.lock

Then purged any such build directory.

As user servo in the buildbot directory. Discovered a few additional ones we hadn't hit yet and cleaned them up.

cc @edunham @aneeshusa