Fix whitespace
Fix incorrect column name in psql \d code.

pg_index.indisreplident had at one time in its development been called
indisidentity.  describe.c got missed when it was renamed.
Bug introduced in commit 07cacba983ef79be4a84fcd0e0ca3b5fcb85dd65.

Andres Freund
Add a regression test case for \d on an index.

Previous commit shows the need for this.  The coverage isn't really
thorough, but it's better than nothing.