Fix `git svn` `rebase` & `dcommit` if top-level HEAD directory exist

When a file (or a directory) called HEAD exists in the working tree,
internal calls git svn makes trigger "did you mean a revision or a
path?" ambiguity check.

    $ git svn rebase
    fatal: ambiguous argument 'HEAD': both revision and filename
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'
    rev-list --first-parent --pretty=medium HEAD: command returned error: 128

Explicitly disambiguate by adding "--" after the revision.

Signed-off-by: Slava Kardakov <ojab@ojab.ru>
Reviewed-by: Jeff King <peff@peff.net>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>