Before this fix, doing CTRL-C and CTRL-V on a file in Windows explorer
caused the **source** and the the destination to be truncated to 0.
This is because Windows opens the source file with Create with flags
`O_RDWR|O_CREATE|O_EXCL` but doesn't write to it - it only reads from
it. Rclone was taking the call to Create as a signal to always make a
new file, but this is incorrect.
This fix reads an existing file from the directory if it exists when
Create is called rather than always creating a new one. This fixes the
problem.
Fixes#5181