Indentation is incorrect for joins in formatted physical plans

[andygrove]

Describe the bug
The new physical plan formatting code does not handle joins correctly. Both children of a join should be shown at the same indent level.

Here is an example of an incorrectly formatted plan:

ballista-executor_1   |     HashAggregateExec: mode=Partial, gby=[l_shipmode], aggr=[SUM(CASE WHEN #o_orderpriority Eq Utf8("1-URGENT") Or #o_orderpriority Eq Utf8("2-HIGH") THEN Int64(1) ELSE Int64(0) END), SUM(CASE WHEN #o_orderpriority NotEq Utf8("1-URGENT") And #o_orderpriority NotEq Utf8("2-HIGH") THEN Int64(1) ELSE Int64(0) END)]
ballista-executor_1   |       CoalesceBatchesExec: target_batch_size=4096
ballista-executor_1   |         HashJoinExec: mode=CollectLeft, join_type=Inner, on=[("l_orderkey", "o_orderkey")]
ballista-executor_1   |           CoalesceBatchesExec: target_batch_size=4096
ballista-executor_1   |             FilterExec: l_shipmode IN ([Literal { value: Utf8("MAIL") }, Literal { value: Utf8("SHIP") }]) AND l_commitdate < l_receiptdate AND l_shipdate < l_commitdate AND l_receiptdate >= CAST(1994-01-01 AS Date32) AND l_receiptdate < CAST(1995-01-01 AS Date32)
ballista-executor_1   |               CsvExec: source=Path(/data/lineitem.tbl: [/data/lineitem.tbl]), has_header=false
ballista-executor_1   |                 CsvExec: source=Path(/data/orders.tbl: [/data/orders.tbl]), has_header=false

To Reproduce
Run the integration tests and look at the logs (once #344 is merged).

Expected behavior
Both children of a join should be shown at the same indent level.

Additional context
Feature was added in #337

[andygrove]

@alamb fyi