The activity graph doesn't show a diff for the diffing track in the comparison view

STR:

1. open [this comparison profile](https://profiler.firefox.com/compare/?globalTrackOrder=0-1-2&profiles[]=https%3A%2F%2Fprofiler.firefox.com%2Fpublic%2Fbf89d8af7ead8296ab59ad51a0b25acef7d18425%2Fmarker-chart%2F%3FglobalTrackOrder%3D9-0-1-2-3-4-5-6-7-8%26hiddenGlobalTracks%3D1-2-3-4-5-7%26hiddenLocalTracksByPid%3D29369-1%26localTrackOrderByPid%3D4411-1-0~29588-0~29567-0~19549-0~19528-0~30211-0~29369-2-0-1~29522-0~29476-0~%26thread%3D11%26v%3D4&profiles[]=https%3A%2F%2Fprofiler.firefox.com%2Fpublic%2Fbf89d8af7ead8296ab59ad51a0b25acef7d18425%2Fmarker-chart%2F%3FglobalTrackOrder%3D9-0-1-2-3-4-5-6-7-8%26hiddenGlobalTracks%3D1-2-3-4-5-7%26hiddenLocalTracksByPid%3D29369-1%26localTrackOrderByPid%3D4411-1-0~29588-0~29567-0~19549-0~19528-0~30211-0~29369-2-0-1~29522-0~29476-0~%26thread%3D11%26v%3D4&thread=0&v=5). This is a diff of twice the same profile, so the activity graph should be seen empty.

But it's not empty. I think that for the activity graph we see a "concatenation" of both profiles instead of a diff.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-331)