Remove prompt prefix from completion result

[coffebar]

When I write "# commit dotfiles" and press C-x, my terminal looks like this:

➜  ~ # commit dotfiles
#!/bin/zsh

# commit dotfiles
git add . && git commit -m "Update dotfiles"

If you merge this PR, it will look like this:

➜  ~ # commit dotfiles
git add . && git commit -m "Update dotfiles"

[tom-doerr]

Thank you! Good idea to limit the replace count to 1