Add option to prevent checking git repository status when accessing repo over a slow connection.

Currently when one has an network share mounted, navigating into directories containing git repositories can be extremely slow.

This modification allows one to specify in one's repository configuration a single hostname in the `oh-my-zsh.show-branch-on-only` configuration key that ZSH will check prior to checking a repository's status.  Should this configuration setting be specified, and the hostname in its value not match one's hostname, repository status will not be checked.