batch check whether submodule needs pushing into one call

batch check whether submodule needs pushing into one call

We run a command for each sha1 change in a submodule. This is
unnecessary since we can simply batch all sha1's we want to check into
one command. Lets do it so we can speedup the check when many submodule
changes are in need of checking.

Signed-off-by: Heiko Voigt <redacted>
Signed-off-by: Junio C Hamano <redacted>