When getting a list of branches but the file doesn't exist, print a
custom error message that informs users that it's necessary to run
`git-checkout-store` before `git-checkout-history` will be able to do
anything.

Use a different history list for each repo.

Now the YAML file is organised in this way:

    /path/to/repo:
      - branch name
      - branch name
    /path/to/another/repo:
      - branch name

A note and installation instructions for shell completion of
`git-checkout-store`.

Add a patch file that can be used on git's git-completion.bash file.
This gives `git-checkout-store` the same shell completion provided to
`git checkout`.

For those accustomed to git shell completion, this should make
`git-checkout-store` easier to use.

The repository directory location string had a trailing newline at the
end which I don't want because this is going to be the key of a hash, so
it doesn't make sense to have it.

Query git to get the current repo path. I plan on using that string as
the key for branches.

This will allow us to only store and query checkout history for the
current repository.

TODO: get rid of trailing newline