Occasional git-copy-diffs "failed to lock" remote error

[ryan-williams]

I've seen this just a few times:

$ gcd
Got remote dev3 at dev3:c/internal-tools. Pushing..
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Counting objects: 170, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (166/166), done.
Writing objects: 100% (170/170), 1.09 MiB | 0 bytes/s, done.
Total 170 (delta 93), reused 0 (delta 0)
error: Ref refs/remotes/origin/master is at 64ac2a8acf9e743eb5c9c92230139b5ed2f7bd67 but expected 47f493f8a8ecf3d0d3c21426434330e51e2f5ee9
remote: error: failed to lock refs/remotes/origin/master
To dev3:c/internal-tools
   47f493f..970ac6d  master -> master
   47f493f..64ac2a8  origin/HEAD -> origin/HEAD
 * [new branch]      docs -> docs
 * [new branch]      origin/docs -> origin/docs
 * [new branch]      origin/hamkepi-processors -> origin/hamkepi-processors
 * [new branch]      origin/ketrew_master -> origin/ketrew_master
 * [new branch]      origin/pr/79 -> origin/pr/79
 * [new branch]      origin/pr/84 -> origin/pr/84
 * [new branch]      origin/pr/89 -> origin/pr/89
 * [new branch]      origin/pr/91 -> origin/pr/91
 ! [remote rejected] origin/master -> origin/master (failed to lock)
error: failed to push some refs to 'dev3:c/internal-tools'
Traceback (most recent call last):
  File "/Users/ryan/c/git-helpers/remote/git-copy-diffs", line 29, in <module>
    subprocess.check_output(['git', 'push', remote.name])
  File "/Users/ryan/anaconda/lib/python3.4/subprocess.py", line 620, in check_output
    raise CalledProcessError(retcode, process.args, output=output)
subprocess.CalledProcessError: Command '['git', 'push', 'dev3']' returned non-zero exit status 1

Running gcd again typically succeeds.