bashrc: Fix PROMPT_COMMAND on Mac

When Terminal.app is configured to cd into the cwd when opening new tabs
and windows, it sets PROMPT_COMMAND='update_terminal_cwd;'. When
~/.bashrc appends the custom PROMPT_COMMAND to the one pre-defined by
Terminal.app, it must take care to not insert successive ;'s into
PROMPT_COMMAND like this:
PROMPT_COMMAND='... update_terminal_cwd; ; ...'. That's a bash syntax
error.
bashrc,git: Fix inclusion of git-prompt.sh on MacPorts

Search for git-prompt.sh in /opt/local/share/git-core.