Improve performance of PR import

What does this MR do?

The merge_requests.reload.find makes AR to load ALL Merge Requests for given project. This results in PullRequestImporter to have O(n) complexity and memory pressure (n - number of already imported PR).

The .reload is not needed as the .find always perform SQL query to find given object.

After that change importing pull requests from https://github.com/gitlabhq/gitlabhq have the very constant time between 0.01-1s for me, instead of 10-40s at the end of the process.

This happens for the following reason:

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

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