git-branch-list: Verify ID is an integer before doing `checkout`

When running:

    $ git branch-list 2

we want to ensure the argument "2" in the example above is an integer.
If so, we take it to represent a branch-list ID. Otherwise an error code
should be returned.

Thanks to Peter Ho on StackOverflow for the neat Bash integer test:
https://stackoverflow.com/questions/2210349/bash-test-whether-string-is-valid-as-an-integer/19116862#19116862

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