Remove DB transaction from Rebase Gitaly operation

What does this MR do?

It removes a transaction that was enclosing an expensive Gitaly operation (Rebase). I was able to reproduce locally, and we were seeing this when it surpassed the idle_in_transaction_session_timeout DB setting:

2019-11-25_19:30:48.04046 rails-background-jobs : 2019-11-25T19:30:48.040Z 84652 TID-ovz5hpums RebaseWorker JID-289d814f770bd2faad9a4159 INFO: start
2019-11-25_19:30:48.23688 rails-background-jobs : 2019-11-25T19:30:48.236Z 84652 TID-ovz5hpums RebaseWorker JID-289d814f770bd2faad9a4159 INFO: arguments: [37,1]
2019-11-25_19:31:03.59492 rails-background-jobs : FATAL:  terminating connection due to idle-in-transaction timeout
2019-11-25_19:31:03.60105 rails-background-jobs : 2019-11-25T19:31:03.600Z 84652 TID-ovz5hpums RebaseWorker JID-289d814f770bd2faad9a4159 INFO: fail: 15.561 sec
2019-11-25_19:31:03.60109 rails-background-jobs : 2019-11-25T19:31:03.601Z 84652 TID-ovz5hpums WARN: {"context":"Job raised exception","job":{"class":"RebaseWorker","args":[37,1],"retry":3,"queue":"rebase","backtrace":true,"jid":"289d814f770bd2faad9a4159","created_at":1574710248.0318432,"correlation_id":"u2Ri7Qp0K52","uber-trace-id":"5ee8f57aacd4796:33d77a7051b6a180:394388d47db97b8:1","enqueued_at":1574710248.0388129},"jobstr":"{\"class\":\"RebaseWorker\",\"args\":[37,1],\"retry\":3,\"queue\":\"rebase\",\"backtrace\":true,\"jid\":\"289d814f770bd2faad9a4159\",\"created_at\":1574710248.0318432,\"correlation_id\":\"u2Ri7Qp0K52\",\"uber-trace-id\":\"5ee8f57aacd4796:33d77a7051b6a180:394388d47db97b8:1\",\"enqueued_at\":1574710248.0388129}"}
2019-11-25_19:31:03.60110 rails-background-jobs : 2019-11-25T19:31:03.601Z 84652 TID-ovz5hpums WARN: ActiveRecord::StatementInvalid: PG::UnableToSend: no connection to the server
2019-11-25_19:31:03.60112 rails-background-jobs : : UPDATE "merge_requests" SET "rebase_jid" = $1 WHERE "merge_requests"."id" = $2
2019-11-25_19:31:03.60113 rails-background-jobs : 2019-11-25T19:31:03.601Z 84652 TID-ovz5hpums WARN: /Users/osw/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/activerecord-5.2.3/lib/active_record/connection_adapters/postgresql_adapter.rb:611:in `async_exec_params'

Instead, now we rescue from possible Gitaly errors, clean the rebase_commit_sha column then raise the error back.

More information can be found at #33943 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
• Database guides
• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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