Fix NoMethodError when deleting merged branches via API

Vasilii Iakliushinrequested to merge
568854-fix-no-method-error into master

What does this MR do and why?

Contributes to #568854 (closed)

Problem

DELETE ':id/repository/merged_branches' requires push permissions to the project but they are not enforced.

Solution

Add missing push_code permissions check

References

NoMethodError when deleting merged branches via... (#568854 - closed)

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Related to #568854 (closed)

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