git-prompt: preserve value of $? inside shell prompt

git-prompt: preserve value of $? inside shell prompt

If you have a prompt which displays the command exit status,
__git_ps1 without this change corrupts it, although it has
the correct value in the parent shell:

	~/src/git (master) 0 $ set | grep ^PS1
	PS1='\w$(__git_ps1) $? \$ '
	~/src/git (master) 0 $ false
	~/src/git (master) 0 $ echo $?
	1
	~/src/git (master) 0 $

There is a slightly ugly workaround:

	~/src/git (master) 0 $ set | grep ^PS1
	PS1='\w$(x=$?; __git_ps1; exit $x) $? \$ '
	~/src/git (master) 0 $ false
	~/src/git (master) 1 $

This change makes the workaround unnecessary.

Signed-off-by: Tony Finch <dot@dotat.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>