Support CI for matching branches on forks

Currently, people with push access to the main Riot repos can push matching
branch names to Riot and the SDKs, and CI will test all the branches together.
This change allows contributors to access the same ability when submitting
several matching PRs from their fork of each repo.

Part of #9041

scripts/fetch-develop.deps.sh

@@ -6,7 +6,7 @@
 # the branch the current checkout is on, use that branch. Otherwise,
 # use develop.
 
-set -e
+set -ex
 
 GIT_CLONE_ARGS=("$@")
 [ -z "$defbranch" ] && defbranch="develop"
@@ -41,7 +41,19 @@ function dodep() {
     # Try the target branch of the push or PR.
     # Use the default branch as the last resort.
     if [[ "$BUILDKITE" == true ]]; then
-        clone $org $repo $BUILDKITE_BRANCH ||
+        # If BUILDKITE_BRANCH is set, it will contain either:
+        #   * "branch" when the author's branch and target branch are in the same repo
+        #   * "author:branch" when the author's branch is in their fork
+        # We can split on `:` into an array to check.
+        BUILDKITE_BRANCH_ARRAY=(${BUILDKITE_BRANCH//:/ })
+        if [[ "${#BUILDKITE_BRANCH_ARRAY[@]}" == "2" ]]; then
+            prAuthor=${BUILDKITE_BRANCH_ARRAY[0]}
+            prBranch=${BUILDKITE_BRANCH_ARRAY[1]}
+        else
+            prAuthor=$org
+            prBranch=$BUILDKITE_BRANCH
+        fi
+        clone $prAuthor $repo $prBranch ||
         clone $org $repo $BUILDKITE_PULL_REQUEST_BASE_BRANCH ||
         clone $org $repo $defbranch ||
         return $?