Use run_shell_cmd() to run all git operations.
Use conditional keywords consistently.
Remove warning about forced update.

Forced updates are disabled by default. Since forced updates must be explicitly enabled by the user in the config file, the warning has little value.
Rephrase message.

Do not explicitly use nanoc syntax (which may change) in the message.
Simplify remote processing.

There is not need to get the remote URL, because an exception is raised if the remote alias is not already defined.
Add unit tests for Git deployer.