submodule: clean up substitutions in script

submodule: clean up substitutions in script

'recommend_shallow' and 'jobs' variables do not need quotes. They only hold a
single token value, and even if they were multi-token it is likely we would want
them split at IFS rather than pass a single string.

'progress' is a boolean value. Treat it like the other boolean values in the
script by using a substitution.

Signed-off-by: Casey Fitzpatrick <kcghost@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>