completion: fix prompt with unset SHOWCONFLICTSTATE in nounset mode

`GIT_PS1_SHOWCONFLICTSTATE` is a user variable that might not be set,
causing errors when the shell is in `nounset` mode.

Take into account on access by falling back to an empty string.

Signed-off-by: Ville Skyttä <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: scop

contrib/completion/git-prompt.sh

@@ -528,7 +528,7 @@ __git_ps1 ()
 	fi
 
 	local conflict="" # state indicator for unresolved conflicts
-	if [[ "${GIT_PS1_SHOWCONFLICTSTATE}" == "yes" ]] &&
+	if [[ "${GIT_PS1_SHOWCONFLICTSTATE-}" == "yes" ]] &&
 	   [[ $(git ls-files --unmerged 2>/dev/null) ]]; then
 		conflict="|CONFLICT"
 	fi