How to amend a commit without changing commit message (reusing the previous one)?

git commit -C HEAD --amend will do what you want. The -C option takes the metadata from another commit.

Since Git 1.7.9 you can also use git commit --amend --no-edit to get your result.

Note that this will not include metadata from the other commit such as the timestamp or tag, which may or may not be important to you.

Another (silly) possibility is to git commit --amend <<< :wq if you've got vi(m) as $EDITOR.