PYSEC-2026-3842 GitPython: Environment-variable exfiltration via os.path.expandvars() on Repo.clone_from() URL

Summary Repo.clonefrom passes the caller-supplied remote URL through Git.polishurl, which on every non-Cygwin platform calls os.path.expandvars on the URL before handing it to git clone. An attacker who controls the URL argument — the documented use case for clonefrom in "import repository from...