ci/request-reviews: fix headRef to point to the PR head

Previously, headRef points to the master branch of Nixpkgs, which
basically means no code owner review will be requested.

The problem can be verified using the following command.

$ DRY_MODE=1 ./ci/request-reviews/request-reviews.sh NixOS/nixpkgs 347973 ci/OWNERS
[...]
This PR touches 0 files
Requesting reviews from: {
  "reviewers": []
}
[...]

Additionally, the comment about conflicts is removed thanks to the
unambiguous way of specifying ref.

ci/request-reviews/request-reviews.sh

@@ -60,9 +60,8 @@ git -C "$tmp/nixpkgs.git" remote add fork https://github.com/"$prRepo".git
 git -C "$tmp/nixpkgs.git" config remote.fork.partialclonefilter tree:0
 git -C "$tmp/nixpkgs.git" config remote.fork.promisor true
 
-# Our local branches mirror Nixpkgs, so make sure to not try to update any to avoid conflicts
 git -C "$tmp/nixpkgs.git" fetch --no-tags fork "$prBranch"
-headRef=$(git -C "$tmp/nixpkgs.git" rev-parse HEAD)
+headRef=$(git -C "$tmp/nixpkgs.git" rev-parse refs/remotes/fork/"$prBranch")
 
 log "Checking correctness of the base branch"
 if ! "$SCRIPT_DIR"/verify-base-branch.sh "$tmp/nixpkgs.git" "$headRef" "$baseRepo" "$baseBranch" "$prRepo" "$prBranch" | tee "$tmp/invalid-base-error" >&2; then