Improve instructions around how to set git-prompt preference variables.

When I first tried to use the git-prompt.sh script, I followed the instructions at the top of the file and everything worked, except for the "GIT_PS1_SHOWDIRTYSTATE" preference. Even though I had it set to true, I wasn't seeing the "()" and "(+)" in my shell prompt. After a few hours of poking around, I finally discovered what I had done wrong: I was setting the variable *after the "source ~/.git-prompt.sh" line, and it has to be set before.

I moved the lines around my .bashrc script and got everything working, but I thought the instructions could be a bit more explicit about how to set these preference variables, especially because this is a silent failure situation; the preference doesn't work if you get the order wrong and there are no warnings or errors to guide you.

This patch started as a PR on the official Github project two years ago (
https://github.com/git/git/pull/425), back when I didn't know the official process for submitting patches. Luckily, a kind user (@dscho) saw it and pointed me in the right direction to get it submitted.

This will be my first time submitting a patch, so hopefully I've figured out the process.

Edward D'Souza (1):
git-prompt.sh: update shell variable instructions.

 contrib/completion/git-prompt.sh | 5 +++++
 1 file changed, 5 insertions(+)

base-commit: cb5918aa0d50f50e83787f65c2ddc3dcb10159fe
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-255%2Fghedsouza%2Fpatch-1-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-255/ghedsouza/patch-1-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/255

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