diff-index.c: "git diff" has no need to read blob from the standard input

diff-index.c: "git diff" has no need to read blob from the standard input

Only "diff --no-index -" does.  Bolting the logic into the low-level
function diff_populate_filespec() was a layering violation from day
one.  Move populate_from_stdin() function out of the generic diff.c
to its only user, diff-index.c.

Also make sure "-" from the command line stays a special token "read
from the standard input", even if we later decide to sanitize the
result from prefix_filename() function in a few obvious ways,
e.g. removing unnecessary "./" prefix, duplicated slashes "//" in
the middle, etc.

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