git merge commits staged files (when two trees are identical)

Hi everybody,

we just stumbled over a situation in which a merge commits staged changes into the merge commit. This happens when the merged-in branch does have commits ('main') but has the same tree ('--allow-empty') as the merge base:

    git init
    echo eins >a
    git add a
    git commit -m initial
    git branch sub
    git commit -m main --allow-empty
    git checkout sub
    : two
    echo zwei >>a
    git add a
    git commit -m underway
    : three
    echo drei >>a
    git add a              # important
    git status
    git diff --cached
    git merge master -m 'merge'
    git status
    git log --cc -1

If the change isn't staged (comment out the '# important' line) the change survives as unstaged.

That is a bug?

- Andreas