Implement basic tests for critical git functions

[DerNamenlose]

Currently the critical git stuff (everything modifying the history) is tested only by clicking through it while developing. This leaves even critical bugs to slip through the net sometimes (e.g. broken merge code not recording parent commits correctly). The code should contain some baseline tests to check whether the modifying functions like do_merge etc. work correctly.

[DerNamenlose]

Current backend implementation relies on the tauri::Window object being available throughout the code to emit things like index changed etc. This is less than ideal from the testability perspective. Either restructure the relevant code to not rely on the Window object (e.g. by wrapping the notifications in respective callbacks/traits) or implement tests as frontend e2e tests.