graph: add --graph-lane-limit option

graph: add --graph-lane-limit option

Replace the hard-coded lane limit with a user-facing
option '--graph-lane-limit=<n>'. It caps the number of
visible lanes to n. This option requires '--graph', without
it, limiting the graph has no meaning, in this case error out.

Zero and negative values are valid inputs but silently
ignored treating them as "no limit", the same as not using
the option. This follows what '--max-parents' does with
negative values.

The default is 0, same as not being used.

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>