t3600: document failure of rm across symbolic links

t3600: document failure of rm across symbolic links

If we have a symlink "d" that points to a directory, we
should not be able to remove "d/f". In the normal case,
where "d/f" does not exist in the index, we already disallow
this, as we only remove things that git knows about in the
index. So for something like:

  ln -s /outside/repo foo
  git add foo
  git rm foo/bar

we will properly produce an error (as there is no index
entry for foo/bar). However, if there is an index entry for
the path (e.g., because the movement is due to working tree
changes that have not yet been reflected in the index), we
will happily delete it, even though the path we delete from the
filesystem is not the same as the path in the index.

This patch documents that failure with a test.

While this is a bug, it should not be possible to cause
serious data loss with it. For any path that does not have
an index entry, we will complain and bail. For a path which
does have an index entry, we will do the usual up-to-date
content check. So even if the deleted path in the filesystem
is not the same as the one we are removing from the index,
we do know that they at least have the same content, and
that the content is included in HEAD.

That means the worst case is not the accidental loss of
content, but rather confusion by the user when a copy of a
file another part of the tree is removed. Which makes this
bug a minor and hard-to-trigger annoyance rather than a
data-loss bug (and hence the fix can be saved for a rainy
day when somebody feels like working on it).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>