graph: commit and post-merge lines for left-skewed merges

graph: commit and post-merge lines for left-skewed merges

Following the introduction of "left-skewed" merges, which are merges
whose first parent fuses with another edge to its left, we have some
more edge cases to deal with in the display of commit and post-merge
lines.

The current graph code handles the following cases for edges appearing
to the right of the commit (*) on commit lines. A 2-way merge is usually
followed by vertical lines:

        | | |
        | * |
        | |\ \

An octopus merge (more than two parents) is always followed by edges
sloping to the right:

        | |  \          | |    \
        | *-. \         | *---. \
        | |\ \ \        | |\ \ \ \

A 2-way merge is followed by a right-sloping edge if the commit line
immediately follows a post-merge line for a commit that appears in the
same column as the current commit, or any column to the left of that:

        | *             | * |
        | |\            | |\ \
        | * \           | | * \
        | |\ \          | | |\ \

This commit introduces the following new cases for commit lines. If a
2-way merge skews to the left, then the edges to its right are always
vertical lines, even if the commit follows a post-merge line:

        | | |           | |\
        | * |           | * |
        |/| |           |/| |

A commit with 3 parents that skews left is followed by vertical edges:

        | | |
        | * |
        |/|\ \

If a 3-way left-skewed merge commit appears immediately after a
post-merge line, then it may be followed the right-sloping edges, just
like a 2-way merge that is not skewed.

        | |\
        | * \
        |/|\ \

Octopus merges with 4 or more parents that skew to the left will always
be followed by right-sloping edges, because the existing columns need to
expand around the merge.

        | |  \
        | *-. \
        |/|\ \ \

On post-merge lines, usually all edges following the current commit
slope to the right:

        | * | |
        | |\ \ \

However, if the commit is a left-skewed 2-way merge, the edges to its
right remain vertical. We also need to display a space after the
vertical line descending from the commit marker, whereas this line would
normally be followed by a backslash.

        | * | |
        |/| | |

If a left-skewed merge has more than 2 parents, then the edges to its
right are still sloped as they bend around the edges introduced by the
merge.

        | * | |
        |/|\ \ \

To handle these new cases, we need to know not just how many parents
each commit has, but how many new columns it adds to the display; this
quantity is recorded in the `edges_added` field for the current commit,
and `prev_edges_added` field for the previous commit.

Here, "column" refers to visual columns, not the logical columns of the
`columns` array. This is because even if all the commit's parents end up
fusing with existing edges, they initially introduce distinct edges in
the commit and post-merge lines before those edges collapse. For
example, a 3-way merge whose 2nd and 3rd parents fuse with existing
edges still introduces 2 visual columns that affect the display of edges
to their right.

        | | |  \
        | | *-. \
        | | |\ \ \
        | |_|/ / /
        |/| | / /
        | | |/ /
        | |/| |
        | | | |

This merge does not introduce any *logical* columns; there are 4 edges
before and after this commit once all edges have collapsed. But it does
initially introduce 2 new edges that need to be accommodated by the
edges to their right.

Signed-off-by: James Coglan <jcoglan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>