Fix add command in the fake-git example

[milesbarr]

The argv constructed from the parent command should be used for argument parsing.

[coveralls]

Coverage remained the same at 88.261% when pulling 3ef626895dbbee5d6090ff0145f1dc41bb44f900 on milesbarr:patch-1 into ee0de3bc6815ee19d4a46c7eb90f829db0e014b1 on docopt:master.