username/repo support

• You can show an abbreviated username/repo in the prompt by setting GIT_PROMPT_WITH_USERNAME_AND_REPO=1 and setting the placeholder _USERNAME_REPO_ in your GIT_PROMPT_PREFIX. You can also add a GIT_PROMPT_USERNAME_REPO_SEPARATOR=" | " so the username/repo is nicely separated if there is a remote and if there is no remote, neither the username/repo part nor the separator will be shown. See the theme Single_line_username_repo.bgptheme for an example.
• Allow empty GIT_PROMPT_SYMBOLS_NO_REMOTE_TRACKING variable