Fix MR diff file counts for some historic data

What does this MR do?

Some old diffs in the database for GitLab.com are much bigger than current diff limits permit - one example has around 45K records. The files_count value isn't that important, so we can work around this without more schema changes by using (signed) SMALLINT_MAX as a sentinel value.

Ref: #246489 (closed)

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Does this MR meet the acceptance criteria?

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