Changes count is inconsistent
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Background
When base and HEAD diff of a MR has differences (e.g. base diff contains 2 files, HEAD diff contains 1 file), the count shown on the "Changes" tab when the MR page loads will be different once user views the "Changes" tab.
Example MR: patrickbajao/verification!88.
Proposal
Modify Projects::MergeRequestsController#get_diffs_count to get the diff_size of MergeRequest#merge_head_diff.
We have to ensure we only do so if we're actually showing HEAD diff on the changes tab as well because it can show incorrect count for a moment if they're not consistent. We can use the same checks in this code except for the diff_head param check as we don't pass that when we load the MR page.