Date:   Wed, 20 Dec 2017 12:43:10 +0100
From:   Andreas Krey <a.krey@gmx.de>
To:     "git@vger.kernel.org" <git@vger.kernel.org>
Subject: git merge commits staged files (when two trees are identical)
Message-ID: <20171220114310.GA2049@inner.h.apk.li>
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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

-- 
"Totally trivial. Famous last words."
From: Linus Torvalds <torvalds@*.org>
Date: Fri, 22 Jan 2010 07:29:21 -0800