Address reported issues for the detail chart

[eth3lbert]

Fix reported issues #1842 (comment) and #1842 (comment)

[Kobzol]

Looks great. It's surprising how complicated it is to think about all the various situations that can arise in this seemingly simple situation 😅

[eth3lbert]

Ah, I forgot to change the word benchmarked to end 😅

[Kobzol]

The original motivation for the benchmarked name was that mostly you look at the page when you open a perf. result from a PR that was benchmarked very recently, so the word makes sense there. Of course, sometimes you also open it later, and then it doesn't make that much sense, since both the start and end commits are "benchmarked", so end probably makes more sense. But it's a very small thing.