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

Currently when one has an SMB or NFS share mounted, navigating into folders 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.