Add "axes" to diagram output

[mhagger]

As discussed here, add integers along the top and left edges of diagrams showing the column and row indexes corresponding to the diagram entries, and add the names of the two branch tips at the end of the axes:

       0    5   10  13
       |    |    |  |
   0 - ************** - main
       *............|
       *............|
       *............|
       *............|
   5 - *............|
       *............|
   7 - *------------+
       |
     branch

For the top axis, it can be a little bit awkward to keep the index of the final column from running into the index of the last multiple-of-five column. To solve this, I've:

• Right-justified most column indexes, but left-justified the last one.

• Added an extra space and change the tick mark into / if the last column is adjacent to the last multiple-of-five column, like so:

         0    5   10 11
         |    |    | /
     0 - ************ - main
         *??????????|
         *??????????|
         *??????????|
         *??????????|
     5 - *----------+
         |
       branch
  

Fixes: #155

/cc @abitrolly: What do you think of this alternative?

[abitrolly]

It is an improvement over what is available now. Although still inconvenient to my linking, as requires unix-fu to read wide diagrams, and numbers on current frontier is unreadable and need to be visually constructed from the diagram. For #155 I need only two max numbers and direction of branches to understand what is going on. Numbers in between are not as useful. They may help to understand how much is left to rebase, but a separate status block that can be scripted into watchexec command can serve me better. That's my thoughts.

[abitrolly]

@mhagger better merge it. Maybe make it optional if somebody needs to parse the diagram for alternative presentation. But then the --raw format without the axes should omit the legend too.