Allow a path of rev: or id: for get comands

[blakelauer]

The API allows for the download (get) command to use "path": "rev:a1c10ce0dd78" or "path": "id:a4ayc_80_OEAAAAAAAAAYa" but the validatePath function adds "/" to the path if it is not there. This
results in "path": "/rev:a1c10ce0dd78" and an error.

This change stops the "/" from being added if the path begins with "rev:" or "id:". This allows the command dbxcli get rev:a1c10ce0dd78 file.ext to run and download a specific revision of a file.

[diwakergupta]

Thanks @blakelauer ! Can you also update the documentation to reflect this new capability?

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