ensureLink does not ensure the destination is hard linked to the source

[pyrocat101]

• Operating System: macOS 10.15.7
• Node.js version: v12.20.1
• fs-extra version: 5.0.0 (checked source code in master and the issue is still there)

ensureLink shortcuts if the destination path exists, without verifying if the destination file's inode is the same as the source file. This is misleading because it is not really an "ensurance".

Relevant code:

node-fs-extra/lib/ensure/link.js

Line 19 in 6bffcd8

if (destinationExists) return callback(null)

[RyanZim]

I think this should be fixed by #826; would appreciate review/testing there.