Please use "Use Message for Commit" and then the interactive rebase. It highlights equally named commits and offers auto-squash commands.

In the log window of version 19.1 there is a "Use Message for Commit" command.

Should it apply to the whole file or just for new characters?

Right now, we enhanced the interactive rebase to highlight equal named commits, and provide options to squash them.

What is still missing when using Ctrl+P and Ctrl+Shift+P?