Bugfix: GIT_EXTERNAL_DIFF with more than one changed files

Bugfix: GIT_EXTERNAL_DIFF with more than one changed files

When there is more than one file that are changed, running git diff with
GIT_EXTERNAL_DIFF incorrectly diagnoses an programming error and dies.
The check introduced in 479b0ae (diff: refactor tempfile cleanup handling,
2009-01-22) to detect a temporary file slot that forgot to remove its
temporary file was inconsistent with the way the codepath to remove the
temporary to mark the slot that it is done with it.

This patch fixes this problem and adds a test case for it.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>