Remote and submodule creation expanded environment-variable references in
caller-supplied URLs before storing them in Git configuration. This could
expose process environment values through a later network request or a tracked
.gitmodules file.

Preserve variables literally in the remaining untrusted URL callers while
retaining Git.polish_url() expansion for intentional local-path normalization.
Add regression tests covering remote configuration and submodule configuration.

Git baseline: git remote add and git submodule add accept URL arguments
literally; Git does not perform shell-style environment expansion on those
arguments.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

…ration

fix: prevent environment expansion in remote URLs