Doing so is much faster and gives the same output.
Here are some numbers:
$ time git submodule summary
real 0m0.219s
user 0m0.050s
sys 0m0.111s
$ time git diff --submodule
real 0m0.012s
user 0m0.003s
sys 0m0.009s
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>