Make graph/message separator width configurable in renderdag

renderdag appears to hardcode one separator space between the rendered graph and the message text, e.g. @ message: one space from the graph cell, plus one separator before the message.

The hardcoded separator seems to be here:

https://github.com/facebook/sapling/blob/main/eden/scm/lib/renderdag/src/output.rs#L45-L54

and for pad/message continuation lines:

https://github.com/facebook/sapling/blob/main/eden/scm/lib/renderdag/src/output.rs#L58-L70

In Jujutsu (jj), this can feel like wasted horizontal space in jj log, where users may prefer @ message instead.

It would be useful if renderdag exposed an option for the graph/message separator width/content, defaulting to the current behavior, so callers could choose 0/"" or something for more compact/different output.