cmake: do find Git for Windows' shell interpreter

cmake: do find Git for Windows' shell interpreter

By default, Git for Windows does not install its `sh.exe` into the
`PATH`. However, our current `CMakeLists.txt` expects to find a shell
interpreter in the `PATH`.

So let's fall back to looking in the default location where Git for
Windows _does_ install a relatively convenient `sh.exe`:
`C:\Program Files\Git\bin\sh.exe`

Helped-by: Øystein Walle <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>