submodule-config: keep update strategy around

submodule-config: keep update strategy around

Currently submodule.<name>.update is only handled by git-submodule.sh.
C code will start to need to make use of that value as more of the
functionality of git-submodule.sh moves into library code in C.

Add the update field to 'struct submodule' and populate it so it can
be read as sm->update or from sm->update_command.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>