graph: limit the graph width to a hard-coded max

Repositories that have many active branches at the same time
produce wide graphs. A lane consists of two columns, the edge and the
padding (or another edge), each branch takes a lane in the graph and
there is no way to limit how many can be shown.

Limit the graph engine to draw at most 15 lanes. Lanes over the limit are
not rendered.

On the commit line, if the commit lives on a visible lane, show the
normal commit mark and stop rendering. If the commit lives on the
first hidden lane, show the "*" commit mark so it is known that
this commit lives in the first hidden lane. Commits on deeper lanes
aren't rendered, but the commit subject will always remain.

For merges, the post-merge lane is only needed when the commit or
the first parent lives on a visible lane (to draw the connection
between them), when both are on hidden lanes, post-merge carries
no useful information, skip it and go to collapsing or padding state.

Also fix a pre-existing indentation issue.

The hard-coded limit will be replaced by a user-facing option
on a subsequent commit.

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

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