gitk can inadvertently call executables in the worktree

Git for Windows is the Windows port of the revision control system Git. Prior to Git for Windows version 2.39.2, when `gitk` is run on Windows, it potentially runs executables from the current directory inadvertently, which can be exploited with some social engineering to trick users into running untrusted code. A patch is available in version 2.39.2. As a workaround, avoid using `gitk` (or Git GUI's "Visualize History" functionality) in clones of untrusted repositories.