subtree: fix "git subtree split --rejoin"

"git merge" in v2.9 prevents merging unrelated histories.

"git subtree split --rejoin" creates unrelated histories when
creating a split repo from a raw sub-directory that did not
originate from an invocation of "git subtree add".

Restore the original behavior by passing --allow-unrelated-histories
when merging subtrees.  This ensures that the synthetic history
created by "git subtree split" can be merged.

Add a test to ensure that this feature works as advertised.

Reported-by: Brett Cundal <brett.cundal@iugome.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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

@@ -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" &&