(lightweight?) stacked PR support

[adboio]

problem

1. i am currently working on a stack of small bug fixes

• they're all unrelated, no need to actually be a stack, but that's just my preferred workflow

2. i leave ph code to run gt create / gt submit
3. ph code struggles to keep up with what's happening

• wrong PR is often referenced in the header
• diff stats are confusing when going to a downstack task (local diff takes precedence over PR diff)

solution

i'd love to get native stack support in at some point, but in the meantime, how can we clean this up?