cannot spawn C:\ProgramFiles(x86)\Git\cmd\: No such file or directory.

Following Git with SSH on Windows, you see that GIT_SSH should reference the ssh executable, not just its path:

(old msysgit)
set GIT_SSH=C:\Program Files (x86)\Git\bin\ssh.exe

(new 2015 git for Windows)
set GIT_SSH=C:\Program Files\Git\usr\bin\ssh.exe

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