Work around Tcl's default `PATH` lookup

As per https://www.tcl.tk/man/tcl8.6/TclCmd/exec.html#M23, Tcl's `exec` function goes out of its way to imitate the highly dangerous path lookup of `cmd.exe`, but _of course_ only on Windows: If a directory name was not specified as part of the application name, the following directories are automatically searched in order when attempting to locate the application: The directory from which the Tcl executable was loaded. The current directory. The Windows 32-bit system directory. The Windows home directory. The directories listed in the path. The dangerous part is the second item, of course: `exec` _prefers_ executables in the current directory to those that are actually in the `PATH`. It is almost as if people wanted to Windows users vulnerable, specifically. To avoid that, Git GUI already has the `_which` function that does not imitate that dangerous practice when looking up executables in the search path. However, Git GUI currently fails to use that function e.g. when trying to execute `aspell` for spell checking. That is not only dangerous but combined with Tcl's unfortunate default behavior and with the fact that Git GUI tries to spell-check a repository just after cloning, leads to a critical Remote Code Execution vulnerability. Let's override both `exec` and `open` to always use `_which` instead of letting Tcl perform the path lookup, to prevent this attack vector. This addresses CVE-2022-41953. For more details, see https://github.com/git-for-windows/git/security/advisories/GHSA-v4px-mx59-w99c Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>