'git filter-branch' fails complaining about an ambiguous argument, if
a tree-filter renames a path and the new pathname happens to match an
existing object name.

After the tree-filter has been applied, 'git filter-branch' looks for
changed paths by running:

  git diff-index -r --name-only --ignore-submodules $commit

which then, because of the lack of disambiguating double-dash, can't
decide whether to treat '$commit' as revision or path and errors out.

Add that disambiguating double-dash after 'git diff-index's revision
argument to make sure that '$commit' is interpreted as a revision.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Jeff King <peff@peff.net>