Fix PROMPT_COMMAND setGitPrompt cumulation when re-sourcing gitprompt.sh

Repeated calls of:
    source .bash-git-prompt/gitprompt.sh
lead to cumulation:
    PROMPT_COMMAND=setGitPrompt;setGitPrompt;...
rendering everything slow
Merge pull request #55 from Bost/prompt-command

Fix PROMPT_COMMAND setGitPrompt cumulation when re-sourcing gitprompt.sh