scripts/git-submodule.sh: Don't rely on non-POSIX 'read' behaviour

scripts/git-submodule.sh: Don't rely on non-POSIX 'read' behaviour

The POSIX definition of the 'read' utility requires that you
specify the variable name to set; omitting the name and
having it default to 'REPLY' is a bashism. If your system
sh is dash, then it will print an error message during build:

qemu/pc-bios/s390-ccw/../../scripts/git-submodule.sh: 106: read: arg count

Specify the variable name explicitly.

Fixes: fdb8fd8cb915647b ("git-submodule: allow partial update of .git-submodule-status")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230720153038.1587196-1-peter.maydell@linaro.org