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>

contrib/completion/git-prompt.sh

@@ -288,6 +288,7 @@ __git_eread ()
 # In this mode you can request colored hints using GIT_PS1_SHOWCOLORHINTS=true
 __git_ps1 ()
 {
+	local exit=$?
 	local pcmode=no
 	local detached=no
 	local ps1pc_start='\u@\h:\w '
@@ -511,4 +512,7 @@ __git_ps1 ()
 	else
 		printf -- "$printf_format" "$gitstring"
 	fi
+
+	# preserve exit status
+	return $exit
 }