Users can set submodule.$name.update to '!command' which will cause
'command' to be run instead of checkout/merge/rebase. This allows
the user finer-grained control over how the update is done.
The primary motivation for this was interoperability with stgit;
however being able to intercept the submodule update process may
prove useful for integrating with or extending other tools.
Signed-off-by: Chris Packham <judge.packham@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>