Merge branch 'en/prompt-under-set-u'

The bash prompt script (in contrib/) did not work under "set -u".

* en/prompt-under-set-u:
  git-prompt: work under set -u

contrib/completion/git-prompt.sh

@@ -433,8 +433,8 @@ __git_ps1 ()
 	fi
 
 	local sparse=""
-	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
-	   [ -z "${GIT_PS1_OMITSPARSESTATE}" ] &&
+	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE-}" ] &&
+	   [ -z "${GIT_PS1_OMITSPARSESTATE-}" ] &&
 	   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 		sparse="|SPARSE"
 	fi
@@ -543,7 +543,7 @@ __git_ps1 ()
 			u="%${ZSH_VERSION+%}"
 		fi
 
-		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
+		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE-}" ] &&
 		   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
 			h="?"
 		fi