Merge branch 'da/subtree-2.9-regression' into maint

"git merge" in Git v2.9 was taught to forbid merging an unrelated
lines of history by default, but that is exactly the kind of thing
the "--rejoin" mode of "git subtree" (in contrib/) wants to do.
"git subtree" has been taught to use the "--allow-unrelated-histories"
option to override the default.

* da/subtree-2.9-regression:
  subtree: fix "git subtree split --rejoin"
  t7900-subtree.sh: fix quoting and broken && chains

contrib/subtree/git-subtree.sh

@@ -662,6 +662,7 @@ cmd_split()
 		debug "Merging split branch into HEAD..."
 		latest_old=$(cache_get latest_old)
 		git merge -s ours \
+			--allow-unrelated-histories \
 			-m "$(rejoin_msg "$dir" $latest_old $latest_new)" \
 			$latest_new >&2 || exit $?
 	fi

contrib/subtree/t/t7900-subtree.sh

@@ -16,16 +16,16 @@ export TEST_DIRECTORY
 
 subtree_test_create_repo()
 {
-	test_create_repo "$1"
+	test_create_repo "$1" &&
 	(
-		cd $1
+		cd "$1" &&
 		git config log.date relative
 	)
 }
 
 create()
 {
-	echo "$1" >"$1"
+	echo "$1" >"$1" &&
 	git add "$1"
 }
 
@@ -71,12 +71,12 @@ join_commits()
 }
 
 test_create_commit() (
-	repo=$1
-	commit=$2
-	cd "$repo"
-	mkdir -p $(dirname "$commit") \
+	repo=$1 &&
+	commit=$2 &&
+	cd "$repo" &&
+	mkdir -p "$(dirname "$commit")" \
 	|| error "Could not create directory for commit"
-	echo "$commit" >"$commit"
+	echo "$commit" >"$commit" &&
 	git add "$commit" || error "Could not add commit"
 	git commit -m "$commit" || error "Could not commit"
 )
@@ -346,6 +346,22 @@ test_expect_success 'split sub dir/ with --rejoin' '
 	)
  '
 
+next_test
+test_expect_success 'split sub dir/ with --rejoin from scratch' '
+	subtree_test_create_repo "$subtree_test_count" &&
+	test_create_commit "$subtree_test_count" main1 &&
+	(
+		cd "$subtree_test_count" &&
+		mkdir "sub dir" &&
+		echo file >"sub dir"/file &&
+		git add "sub dir/file" &&
+		git commit -m"sub dir file" &&
+		split_hash=$(git subtree split --prefix="sub dir" --rejoin) &&
+		git subtree split --prefix="sub dir" --rejoin &&
+		check_equal "$(last_commit_message)" "Split '\''sub dir/'\'' into commit '\''$split_hash'\''"
+	)
+ '
+
 next_test
 test_expect_success 'split sub dir/ with --rejoin and --message' '
 	subtree_test_create_repo "$subtree_test_count" &&