allow pull --rebase on branch yet to be born

allow pull --rebase on branch yet to be born

When doing a "pull --rebase", we check to make sure that the index and
working tree are clean. The index-clean check compares the index against
HEAD. The test erroneously reports dirtiness if we don't have a HEAD yet.

In such an "unborn branch" case, by definition, a non-empty index won't
be based on whatever we are pulling down from the remote, and will lose
the local change.  Just check if $GIT_DIR/index exists and error out.

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