SHELL

git merge message editor

1) press "i" (i for insert)
2) write your merge message
3) press "esc" (escape)
4) write ":wq" (write & quit)
5) then press enter

git merge with message

$ git merge other-branch -m "Commit Message"

git edit merge commit message

For current Git versions (2020+), just do git rebase -i -r <parent>, 
then replace in the editor merge -C with merge -c.
This will open the merge commit's message in the editor during rebasing,
where you can change it

after git merge it want to commit message

press "i" (i for insert)
write your merge message.
press "esc" (escape)
write ":wq" (write & quit)
then press enter.