Untrusted search path on Windows systems leading to arbitrary code execution

Summary

GitPython is a python library used to interact with Git repositories. When resolving a program, Python/Windows look for the current working directory, and after that the PATH environment. GitPython defaults to use the `git` command, if a user runs GitPython from a repo has a `git.exe` or `git` executable, that program will be run instead of the one in the user's `PATH`. This is more of a problem on how Python interacts with Windows systems, Linux and any other OS aren't affected by this. But probably people using GitPython usually run it from the CWD of a repo. An attacker can trick a user to download a repository with a malicious `git` executable, if the user runs/imports GitPython from that directory, it allows the attacker to run any arbitrary commands. There is no fix currently available for windows users, however there are a few mitigations. 1: Default to an absolute path for the git program on Windows, like `C:\\Program Files\\Git\\cmd\\git.EXE` (default git path installation). 2: Require users to set the `GIT_PYTHON_GIT_EXECUTABLE` environment variable on Windows systems. 3: Make this problem prominent in the documentation and advise users to never run GitPython from an untrusted repo, or set the `GIT_PYTHON_GIT_EXECUTABLE` env var to an absolute path. 4: Resolve the executable manually by only looking into the `PATH` environment variable.