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>