clean-cargo-cache fails if no builds have been performed

[jdm]

Error running mach:

    ['clean-cargo-cache', '--keep', '3', '--force']

The error occurred in the implementation of the invoked mach command.

This should never occur and is likely a bug in the implementation of that
command. Consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

OSError: [Errno 2] No such file or directory: '/home/servo/.cargo/git/db'

  File "/home/servo/buildbot/slave/linux-rel-wpt/build/python/servo/bootstrap_commands.py", line 240, in clean_cargo_cache
    git_db_list = filter(lambda f: not f.startswith('.'), os.listdir(git_db_dir))

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄