Re: "git commit --amend --only --" nevertheless commits staged changes

[Jeff King <peff@peff.net>]

On Tue, Jul 10, 2012 at 12:41:13PM +0200, Marc Strapetz wrote:

> When using "git commit --amend --only --message <message> --", I'd
> expect to have just the commit message of my last commit changed,
> according to the man page:
> 
> "--only Make a commit only from the paths specified on the command line,
> disregarding any contents that have been staged so far. [...] If this
> option is specified together with --amend, then no paths need to be
> specified, which can be used to amend the last commit without committing
> changes that have already been staged."
> 
> However, all staged changes are committed as well. So looks like either
> the man page or Git is wrong here!?

I think git has a bug. As far as I can tell, this has never worked as
the documentation advertised. We originally forbid the use of "--only"
without paths as nonsensical. This was loosened by 6a74642 (git-commit
--amend: two fixes., 2006-04-20) to let "--amend --only --", but I don't
think it even worked then.

Using this test:

  git init repo &&
  cd repo &&
  echo "foo one" >foo &&
  echo "bar one" >bar &&
  git add . &&
  git commit -m one &&
  echo "foo two" >foo &&
  echo "bar two" >bar &&
  git add foo &&
  GIT_EDITOR=true git commit --amend -o &&
  git cat-file -p HEAD:foo &&
  git cat-file -p HEAD:bar

I always get:

  foo two
  bar one

i.e., we accidentally amend the commit with the staged contents in the
index. I get the same results for 6a74642 and on. If you switch the
commit to "-o bar", it does work properly (you get the updated "bar",
but the staged "foo" in the index is ignored).

-Peff