'arcyd add-repo' auto substitutes '.' for '-'

Make it easier to handle repositories with dots in their urls by
automatically substituting then for dashes when guessing a name for the
corresponding repo config.

Add doc test to show expected result.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ..
    $ git init --bare origin3.1.git
    $ git clone origin3.1.git/ dev3
    $ cd dev3
    $ git commit --allow-empty -m 'hello'
    $ git push origin master
    $ cd ../arcyd2/
    $ arcyd add-repo localhost fs1 origin3.1.git
    $ ls config/repository/