stg diff / files: don't update directory cache

Do git-update-cache only when diffing with the working tree, not otherwise.
Spending something like 1min for a stg files is bad - yes, my laptop
was really busy and the Linux tree was probably cache-cold, but that's just
not needed.

Also, in diffstat we currently do it both by hand and by calling git.diff.
And in files there's no need at all for that - even the comments says that
"files" has only to do with committed changes.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>