[Feature] Open the diffview window in a floating window instead of a new tab

[LintaoAmons]

Most of the cases, I just need to close the window after I finish git related stuffs and back to where I was before I trigger Diffview.

Currently Diffview will bring me to a new tab, once I close the new diffview tab, I maybe not in the same tab I was before.

e.g.

I have tab 1 | 2 | 3 each of them is one project, and I'm current at tab 1 which is project 1. DiffviewOpen will create a new tab 4, 1 | 2 | 3 | 4. So after I finished and close tab 4， I'm back the tab 3 instead of 1.

A floating window maybe a good alternative, after finished, just need to close the window.