diff-index.c: unify handling of command line paths

diff-index.c: unify handling of command line paths

Regardless of where in the directory hierarchy you are, "-" on the
command line means the standard input.  The old code knew too much
about how the low level machinery uses paths to read from the
working tree and did not bother to have the same check for "-" when
the command is run from the top-level.

Unify the codepaths for subdirectory case and toplevel case into one
and make it clearer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>