Uncontrolled search for the Git directory in Git for Windows

Summary

Git for Windows is a fork of Git containing Windows-specific patches. This vulnerability affects users working on multi-user machines, where untrusted parties have write access to the same hard disk. Those untrusted parties could create the folder `C:\.git`, which would be picked up by Git operations run supposedly outside a repository while searching for a Git directory. Git would then respect any config in said Git directory. Git Bash users who set `GIT_PS1_SHOWDIRTYSTATE` are vulnerable as well. Users who installed posh-gitare vulnerable simply by starting a PowerShell. Users of IDEs such as Visual Studio are vulnerable: simply creating a new project would already read and respect the config specified in `C:\.git\config`. Users of the Microsoft fork of Git are vulnerable simply by starting a Git Bash. The problem has been patched in Git for Windows v2.35.2. Users unable to upgrade may create the folder `.git` on all drives where Git commands are run, and remove read/write access from those folders as a workaround. Alternatively, define or extend `GIT_CEILING_DIRECTORIES` to cover the _parent_ directory of the user profile, e.g. `C:\Users` if the user profile is located in `C:\Users\my-user-name`.