Fix auto-MR-close text from branch name

What does this MR do?

Fix the MR-auto-close feature if a branch name begins with an issue IID.

Are there points in the code the reviewer needs to double check?

It's one line, so check that line thoroughly

Why was this MR needed?

When I create a new MR from a branch starting with an issue IID, the description doesn't get suffixed with the 'Closes #blahblah' that it should do.

What are the relevant issue numbers?

Don't think there are any.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added

This is why this is needed (the form helpers use the _before_type_cast variants where available.

[1] pry(main)> mr = MergeRequest.new
=> #<MergeRequest:0x007fcf28395d88 ...>
[2] pry(main)> mr.description = 'foo'
=> "foo"
[3] pry(main)> mr.description << ' bar'
=> "foo bar"
[4] pry(main)> mr.description
=> "foo bar"
[5] pry(main)> mr.description_before_type_cast
=> "foo"
[6] pry(main)> mr.description += ' bar'
=> "foo bar bar"
[7] pry(main)> mr.description_before_type_cast
=> "foo bar bar"

Milestone changed to %8.9

Added ~149423 label

Status changed to merged

mentioned in commit a3a8798a

Picked into 8-9-stable, will go into 8.9.1.

Removed ~149423 label

mentioned in issue #18970 (closed)

mentioned in commit 9be991df