When diff-no-index is given a relative path to a file outside the repository, it aborts with error. However, if the file is given using an absolute path, the diff runs as expected. The two cases should be treated the same.
Tests and commit message by Tim Henigan.
Signed-off-by: Jeff King <email@example.com> Signed-off-by: Tim Henigan <firstname.lastname@example.org> Signed-off-by: Junio C Hamano <email@example.com>