overwrites existing $PROMPT_COMMAND

[laur89]

Sourcing gitprompt.sh replaces prompt command, not appends to it; following in ~/.bashrc:

$ echo "pre prompt: [$PROMPT_COMMAND]"
$ source "/installation/path/bash-git-prompt/gitprompt.sh"
$ echo "post prompt: [$PROMPT_COMMAND]"

...produces this:

pre prompt: [history -a]
post prompt: [setLastCommandState]

I would expect existing value to survive the import.

Running v 0bca406

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Could this be the cause for issues like #496?