rB546314fc9669 broke `svn_libraries_base_url` utils API compatibility
for no good reasons, making new `branch` argument explicitely optional.
Reported on chat by Michael Kowalski (@makowalski), thanks!
Needed for studio sprite-fright frozen branch.
Also do not overwrite branch for git sub-modules when it is defined, and
fallback to `master` branch in case specified branch is not found in a
specific sub-repository.
This prints a more informative message, and is convenient when working with
local changes or in a branch where you only need to update submodules or tests.