Change of pin branch may fail if checkout becomes dirty

[mosteo]

Originally submitted to mosteo/aaa#7

We have observed some issues when updating branch pins on crate aaa. Should the config/aaa_config.* files be .gitignoreed?

To reproduce:

• alr init xxx
• cd xxx
• alr pin aaa --use=https://github.com/mosteo/aaa --branch=master
• alr build
• Edit the pin in alire.toml, changing branch='master' to branch='feat/machine-output'
• alr update
• Observe

  error: Your local changes to the following files would be overwritten by checkout:
        config/aaa_config.ads
        config/aaa_config.gpr
        config/aaa_config.h
  Please commit your changes or stash them before you switch branches.