On Windows `code` works in the shell, but not when specified to jj, because the
executable is actually named `code.cmd`! This is a feature of Windows, where
certain file extensions (defined by the PATHEXT environment variable) are
automatically treated as executable and can have their extensions omitted.
Presumably, jj doesn't support this feature on Windows, though perhaps it
should. For now, avoid adding an explanation of PATHEXT so as to not clutter
things too much, and just suggest `code.cmd` on Windows. Most readers should be
able to put two and two together from there.
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