[6/n][sl][github][gh stacks] wire up gh stacks to sl pull

[raydatray]

ctx

sl pr pull links a diff's ancestors by parsing the sapling stack list footer, which the stacked workflow omits in #1395

changes made

• when the body has no footer, query the native stacks API and link the ancestors below the diff from the stack instead
• warns only if the diff is in no stack at all

test plan

• next test-ext-github-pr-pull-stacked.t with mocked stack responses
• dogfooded on my own computer

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
• rebase and merge as stack 197 and 198 - they merge as a stack and github doenst ask you to rebase (like it normally would in the previous pr workflow)

• go back to local and run sl pull - we see what those two are marked as landed

• rebase those two remaining diffs on main, and then resubmit, we see that the stack remains preserved on github

---

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 D114402759. (Because this pull request was imported automatically, there will not be any future comments.)