doc: command line interface (cli) dot-repository dwimmery

The Git cli will accept dot '.' (period) as the relative path,
and thus the current repository. Explain this action.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Doc URLs: relative paths imply the dot-respository

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Doc: remote set-head long options

Signed-off-by: Philip Oakley <philipoakley@iee.org>