This more specific name improves MSYS2 detection by applications that
are sensitive to the presence of this file, such as the installer script
make-msys2-installer.bat.
These batch files used to contain duplicated code that has been merged
into start_shell.cmd. Shortcuts can replace them like this:
* Call `start_shell.cmd -mingw32` instead of mingw32_shell.bat.
* Call `start_shell.cmd -mingw64` instead of mingw64_shell.bat.
* Call `start_shell.cmd -msys` instead of msys2_shell.bat.
