graph: wrap cascading commits after 4 columns

graph: wrap cascading commits after 4 columns

Currently the visual root commits in a graph cascade indefinitely until
a commit which is not a visual root or the last commit appears.
On filters like --author where one author might contribute mostly on
single patches this can become a visual issue.

Make the cascading wrap after 4 columns.

There are two possible cases of the wrap:

1. No ambiguity:

* A
  * B
    * C
      * D
* E
  * F

2. Ambiguous conflict:

If F happens to not be a visual root and E gets wrapped back to the
initial column then E and F would be vertically adjacent. The solution
is to forcefully indent E one level:

* A
  * B
    * C
      * D
  * E
* F
* F

The magic number 4 comes as the minimum number of columns to wrap where
the output shows clearly the commits are unrelated and doesn't cause too
much "pyramid" effects

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