t/lib-submodule-update.sh: allow local submodules

t/lib-submodule-update.sh: allow local submodules

To prepare for changing the default value of `protocol.file.allow` to
"user", update the `prolog()` function in lib-submodule-update to allow
submodules to be cloned over the file protocol.

This is used by a handful of submodule-related test scripts, which
themselves will have to tweak the value of `protocol.file.allow` in
certain locations. Those will be done in subsequent commits.

Signed-off-by: Taylor Blau <me@ttaylorr.com>