Add changes_count attribute to all-merge-requests API

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Proposal

The API to get a single merge request returns the changes_count attribute. However, the APIs to get all merge requests for a group or project do not return this attribute. Users who wish to check this attribute for all merge requests in a group or project must call an additional endpoint to get the data.

It appears the attribute was originally added in gitlab-foss!15178 (merged).

Edited by 🤖 GitLab Bot 🤖