Previously, the graphing code printed lines coming out of a merge commit
for all of its parents, even if some of them were uninteresting.  Now it
only prints lines for interesting commits.

For example, for a merge commit where only the first parent is
interesting, the code now prints:

  *  merge commit
  *  interesting child

instead of:

  M  merge commit
  |\
  *  interesting child
Signed-off-by: NAdam Simpkins <adam@adamsimpkins.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

The graphing code had a bug that caused it to output branch lines
incorrectly after ignoring an uninteresting commit.  When computing how
to match up the branch lines from the current commit to the next one, it
forgot to take into account that it needed to initially start with 2
empty spaces where the missing commit would have gone.

So, instead of drawing this,

  | * | <- Commit with uninteresting parent
  |  /
  * |

It used to incorrectly draw this:

  | * | <- Commit with uninteresting parent
  * |
Signed-off-by: NAdam Simpkins <adam@adamsimpkins.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

This change improves the calculation of the amount of horizontal
padding, so that there is always exactly 1 space of padding.
Previously, most commits had 3 spaces of padding, but commits that
didn't have any children in the graph had only 1 space of padding.
Signed-off-by: NAdam Simpkins <adam@adamsimpkins.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

This new API allows the commit history to be displayed as a text-based
graphical representation.
Signed-off-by: NAdam Simpkins <adam@adamsimpkins.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>