Git: Create a fixup commit from the graph view

[hannah-computer]

Currently, creating a fixup commit might look like this:

1. Stage some changes
2. Find the commit to fixup, right click "copy commit ID"
3. In the terminal, type git commit --fixup=, paste the ID, hit enter.

This can get tedious, so it'd be great to have a "Create fixup commit" option combining (2) & (3) into one operation that doesn't involve the terminal.

[hannah-computer]

Found an extension which tries to make this easier teogeb/git-fixup#13 but that is blocked by #201997