make __git_ps1 accept a third parameter in pcmode
make __git_ps1 accept a third parameter in pcmode

The optional third parameter when __git_ps1 is used in
PROMPT_COMMAND mode as format string for printf to further
customize the way the git status string is embedded in the
user's PS1 prompt.

Signed-off-by: Simon Oosthoek <65a59bba37a01727c168656b2046877d95c301eb@xs4all.nl>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git