Fix previous deployment fetches wrong deployment

What does this MR do?

While we were investigating #321032 (closed), we realized that the previous_deployment method actually doesn't return the previous deployment. It actually returns the other-but-latest deployment.

This MR fixes the outstanding issue.

Related #321032 (closed)

Query Performance implication

!58567 (comment 545092453)

@mattkasa That's good question. This change actually also optimizes the query performance.

Here is the current plan:

 Limit  (cost=0.57..4.45 rows=1 width=139) (actual time=8.236..8.238 rows=0 loops=1)
   Buffers: shared read=4
   I/O Timings: read=8.181
   ->  Index Scan using index_deployments_on_environment_id_and_id on public.deployments  (cost=0.57..667.53 rows=172 width=139) (actual time=8.233..8.234 rows=0 loops=1)
         Index Cond: (deployments.environment_id = 39)
         Filter: ((deployments.id <> 124) AND ((deployments.ref)::text = 'master'::text))
         Rows Removed by Filter: 0
         Buffers: shared read=4
         I/O Timings: read=8.181

and here is the new plan:

 Limit  (cost=0.57..3.59 rows=1 width=139) (actual time=8.884..8.885 rows=0 loops=1)
   Buffers: shared hit=3 read=4
   I/O Timings: read=8.755
   ->  Index Scan using index_deployments_on_environment_id_status_and_id on public.deployments  (cost=0.57..3.59 rows=1 width=139) (actual time=8.881..8.881 rows=0 loops=1)
         Index Cond: ((deployments.environment_id = 39) AND (deployments.status = 2) AND (deployments.id < 124))
         Buffers: shared hit=3 read=4
         I/O Timings: read=8.755

Notice that Filter: ((deployments.id <> 124) AND ((deployments.ref)::text = 'master'::text)) is quite expensive that likely takes long time when an environment has a lot of deployments. This is because we don't have effective index on the ref column.

The new plan performs one index scan with index_deployments_on_environment_id_status_and_id and doesn't perform any sequential filtering. This assures that the query will stay performant regardless of the deployments count on an environment.

Screenshots (strongly suggested)

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