git diff: Don't test submodule dirtiness with --ignore-submodules

git diff: Don't test submodule dirtiness with --ignore-submodules

The diff family suppresses the output of submodule changes when
requested but checks them nonetheless. But since recently submodules
get examined for their dirtiness, which is rather expensive. There is
no need to do that when the --ignore-submodules option is used, as
the gathered information is never used anyway.

Signed-off-by: Jens Lehmann <redacted>
Signed-off-by: Junio C Hamano <redacted>