Rebasing is currently using two prepositions, on and onto in the UI. I suggest we use onto as it is more descriptive in how a rebase looks visually. (We also use into when doing merge conflicts).

Three places were on/onto is used...

• a mention is in the success message banner
• and remaining two are here:

Lastly, we should fix the spacing of the spinning icon and the text in the button:

76d48b9, mac