$ git commit -a --amend
[WARNING] Unstaged files detected.
[INFO] Stashing unstaged files to /nail/home/rkwills/.pre-commit/patch1450735701.
An unexpected error has occurred: CalledProcessError: Command: ['git', 'checkout', '--', '.']
Return code: 128
Expected return code: 0
Output: (none)
Errors:
    fatal: Unable to create '/nail/home/rkwills/trees/yelp/pip-faster/.git/index.lock': File exists.

    If no other git process is currently running, this probably means a
    git process crashed in this repository earlier. Make sure no other git
    process is running and remove the file manually to continue.


Check the log at ~/.pre-commit/pre-commit.log

[Mon 12-21 02:08:21PM]$ pip install pre-commit --upgrade
Requirement already up-to-date: pre-commit in ./venv-venv_update/lib/python2.7/site-packages
Cleaning up...

$ ls -l /nail/home/rkwills/trees/yelp/pip-faster/.git/index.lock
ls: cannot access /nail/home/rkwills/trees/yelp/pip-faster/.git/index.lock: No such file or directory