[5/n][sl][github][gh stacks] wire up gh stacks to sl pr submit

[raydatray]

ctx

actually links the submitted PRs into a native github stack and deals with the various constraints github puts on stacked diffs

changes made

• after submit, diffs are linked via the stacks API: create the stack on first submit, then append new diffs when the local stack extends it
• the remote stack is queried before any mutations:
  • if it diverged from the local stack (reorder or dropped diffs), abort before updating bases or pushing. new --restack flag dissolves and recreates it (since the gh API has no reorder operation). tell the user to do this as a ui hint
  • base branches of diffs already in a stack are never updated via the API (since gh rejects that, the stack manages bases on its own). body/title rewrites use the "base-less" mutation from [2/n][sl][github][gh stacks] add support for stack endpoint #1393
• new diffs are created directly against the head branch below them instead of being created against main and re-based afterwards (this is impossible to do once diffs are stacked on gh). falls back to default base for forks, where gh native stacks are not supported and a warning is printed (i wanted to submit this stack with my patch to show it works but alas i cant :( )

test plan

• test-ext-github-submit-stacked.t covers the initial submit, extending an existing stack, diverged stack with/without --restack and with/without local changes to push
• tested live on my own machine/repo

sl config --local extensions.github=/Users/ray/sapling/eden/scm/sapling/ext/github/__init__.py
sl config --local extensions.signing_shim=/Users/ray/.sl-signing-shim.py
sl config --local github.pr-workflow=stacked

• create a stack here [1/n][stacks] test raydatray/rusty-mcrouter#197
• submit 197 and 198 - it creates a stack
• then add 200 locally and submit again - it extends the stack
• then add 202 locally and submit again - it extends the stack
• go down to 198 and amend the diff locally. restack locally and resubmit - modifies only 198 and the stack is updated

---

Stack created with Sapling. Best reviewed with ReviewStack.

• [7/7][sl][github][gh stacks] update some documentation #1398
• [6/n][sl][github][gh stacks] wire up gh stacks to sl pull #1397
• -> [5/n][sl][github][gh stacks] wire up gh stacks to sl pr submit #1396
• [4/n][sl][github][gh stacks] update submit body for stacks #1395
• [3/n][sl][github][gh stacks] add stacked workflow for submit #1394
• [2/n][sl][github][gh stacks] add support for stack endpoint #1393
• [1/n][sl][github][gh stacks] support headers and list params #1392

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D114402728. (Because this pull request was imported automatically, there will not be any future comments.)