add --depth as a configurable option on sdmerge

[alissapajer]

what it says

[sellout]

I would prefer if we could calculate the necessary depth automatically.

But in the mean time, only using --depth when it’s explicitly provided by the sdmerge user sounds reasonable.

[djspiewak]

@sellout There's a chicken-or-egg problem here. It's trivial to infer --depth… once we have the commits. We can't infer --depth until we have the commits, by which point it's too late.

The only solution I can think of would be to iteratively clone things. Start with a low depth, and see if it contains the base hash. If it doesn't, grab the next n * 2 commits. Repeat until we find the base hash.

[sellout]

Well, we could use --shallow-since=<date of base hash>, but I was hoping there was some way to use the base hash directly.