Squash Git Branch Commits in Android Studio

Go to VCS -> Git -> Rebase..., there you can amongst other things check 'Interactive'. At least in the latest IntelliJ IDEA version it is like that and AS is based on IJ. If your AS version is based off an older IJ version, it might be different if possible at all.

Btw. you don't have to use a command line editor when doing interactive rebase from command line. I e. g. have an alias that sets my Git editor to jEdit, so I can decide on each command that needs an editor whether I want to use the command line editor vim, or the GUI tool jEdit. But it can be configured persistently without an alias too of course.

I prefer using the UI too I have found that squash commits can be made in "Rebasing commits" window: "Version Control" -> "Log" -> "Select base commit" -> "Context menu" -> "Interactively Rebase From Here..."