Fix for windows

[kono]

sh> operator of v0.10.0 on Windows platform and compatibility

When specifying "powershell" for shell in a .dig file, the "powershell.exe" was specified as ["powershell.exe","-"] until digdag ver. 0.9.x. Since ver. 0.10 the "-" is no longer necessary.

However, for compatibility, the specification ["powershell.exe","-"] should be read as ["powershell.exe"].

[WIP] Fix sh> operator issue on Windows #1654 is also included.

[yoyama]

I am wondering whether rewriting the task definiton of sh> operator is good way or not. In addition the logic to check condition to rewrite is not solid.

[kono]

@yoyama , thank you for your review. Changed to check more solidly.