rebase -i: let generate_script output to stdout
That makes it harder to miss a non-redirected output, as we can now call
generate_script > "$TODO"
directly.
This cleanup unfortunately makes it necessary to run rev-list twice in
case that there are commits to be picked.
The impact should be low, though, as the expensive flags are not used
in the first call, and the next call has everything in hot cache.
On the upside, generating a "noop" became much faster ;-)
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>