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

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

This isn't an rc file (as we established previously). It's a storage
file. Rename the function to be more clear about it.

Extract code that was duplicated from checkout-history-store so that it
now lives in a single place: the utils.go file.

Add a new function to utils.go that returns the list of branches in
history so that we can grab this for git-checkout-history.

Stop hard-coding the storage filename.

Create utils package and move everything related to branch creation into
the utils.go file. Then call `utils.Store` to store a branch.

Doing this so I can easily store a branch in history from a
`git-checkout-history` call. This is because we want to save to history
even if you're checking out using the history shorthand.