FR: describe mode for editing commit messages in the fzf prompt

[noahmayr]

Is your feature request related to a problem? Please describe.
It's somewhat annoying that describing a commit takes you out of the main ui and opens a full editor.

Describe the solution you'd like
Describe should enter "describe mode" which sets the prompt to the current commit message and updates it when pressing enter

Describe alternatives you've considered
Keep using interactive describe