Use horizontal lines instead of long diagonal lines during the
collapsing state of graph rendering.  For example what used to be:

    | | | | |
    | | | |/
    | | |/|
    | |/| |
    |/| | |
    | | | |

is now

    | | | | |
    | |_|_|/
    |/| | |
    | | | |

This results in more compact and legible graphs.
Signed-off-by: NAllan Caffee <allan.caffee@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>