Improve multiple commits feature

[julieg18]

• Fix bug with dvc exp show -n 3 not always showing the last three commits (see #3035 comment) (Fix missing commit messages in experiment branches #3053)
• Make split between the "main" row and "past commit" rows more obvious (Add "Previous Commits" row to experiments table #3087)
• Add more commit information to table rows in a tooltip (full message, author, git tags, etc.)
• Possibly show the commit messages in quick pick experiment selection (if we can show both columns and commits)
• Show the commit messages in the plots ribbon blocks
• Incorporate the git-commit icon into commit messages

Followup to #3016

[julieg18]

Make split between the "main" row and "past commit" rows more obvious

Looked into some other tables but had some difficulty finding ideas since no table that I had a clear "split" between rows minus when the rows were in collapsible groups. But here's one idea I experimented with:

Though it's hard for me to tell when something looks good or I've been staring at it for too long 😅 Any other ideas that we want to test out? cc @iterative/vs-code, @shcheklein

[shcheklein]

@julieg18 I like it! it's simple and explicit. We'll be able to add a dropdown of some other element to configure which commits to show as a next step, it's an obvious and good place for that.

I don't think we need ":"

[julieg18]

Two design ideas so far for commit tooltips on the experiment table!

Any preference or other ideas/inspiration? cc @iterative/vs-code, @shcheklein

[shcheklein]

I think the first one is better! I can't make screenshot (it doesn't allow me for whatever reason), but take a look at how GitLens represents them in the COMMITS side panel. We can do something similar (simplify if some parts there take more than 10 minutes to implement :))