Proposal: reduce "Unknown reviewer" warnings from delegated r+

[DanielEScherzer]

I figured I would look into some of the "Unknown reviewer" warning

Unknown reviewer: reviewer: "DorianListens", commit: 2836dd15f753a490ea5b89e02c6cfcecd2f32984

corresponds to rust-lang/rust@2836dd1 (merged in as a subtree, originally rust-lang/rust-analyzer@2836dd1) which was in rust-lang/rust-analyzer#12706

• jonas-schievink approves
• bors reports a merge conflict
• jonas-schievink delegates to patch author DorianListens
• patch is rebased
• DorianListens approves

Assuming that we don't want to try and load and analyze the GitHub comment history to figure out what happened, I propose that, in the case of an unknown reviewer, if the merge is of a branch from that user, the warning be omitted

In the linked case, 2836dd15f753a490ea5b89e02c6cfcecd2f32984 had the commit message

Auto merge of #12706 - DorianListens:dscheidt/closure-args, r=DorianListens
    
fix: Extract Function misses locals used in closures
    
This change fixes #12705.
    
In `FunctionBody::analyze`, we need to search any `ClosureExpr`s we encounter
for any `NameRef`s, to ensure they aren't missed.

so we can derive from the name of the forked branch that DorianListens was presumably the author - and is thus already going to be credited as authoring the commit.

This would also work for rollup merges, Unknown reviewer: reviewer: "chbaker0", commit: 342ca46517714ccde6efba95ff552d11b9a92f1c is from rust-lang/rust@342ca46 which was rust-lang/rust#108017 where again the author was delegated to - first line of the merged commit is Rollup merge of https://github.com/rust-lang/rust/pull/108017 - chbaker0:fix-105967, r=chbaker0, again the forked branch indicates the author

Figured I would file an issue for discussion before working on the implementation