Utilize github API to determine target PR branch

This will hopefully resolve issues pertaining to PRs that target
non-develop branches and fail in particular ways (not to mention its not
an accurate test) when rebased against develop

.circleci/config.yml

@@ -8,8 +8,8 @@ jobs:
       - run: apt-get install -y make
       - checkout
       - run:
-          name: Rebase on-top of latest develop
-          command: ./devops/scripts/rebase-develop.sh
+          name: Rebase on-top of github target
+          command: ./devops/scripts/rebase-ci.sh
       - setup_remote_docker
 
       - run: TAG=${CIRCLE_SHA1} make ci-lint-image
@@ -27,8 +27,8 @@ jobs:
       - run: apt-get install -y make
       - checkout
       - run:
-          name: Rebase on-top of latest develop
-          command: ./devops/scripts/rebase-develop.sh
+          name: Rebase on-top of github target
+          command: ./devops/scripts/rebase-ci.sh
       - setup_remote_docker
 
       - restore_cache:
@@ -133,8 +133,8 @@ jobs:
       - checkout
 
       - run:
-          name: Rebase on-top of latest develop
-          command: ./devops/scripts/rebase-develop.sh
+          name: Rebase on-top of github target
+          command: ./devops/scripts/rebase-ci.sh
 
       - run:
           name: Installation apt pre-reqs

devops/scripts/rebase-develop.sh → devops/scripts/rebase-ci.sh

@@ -1,7 +1,12 @@
 #!/bin/bash
+#
+# To be utilized in CircleCI
+# Determine target branch from PR number and rebase against that
 
 set -e
 
+GITHUB_PR_URL="https://api.github.com/repos/freedomofpress/securedrop/pulls/${CIRCLE_PULL_REQUEST##*/}"
+
 # If current branch is not master or doesnt start with release...
 if [[ "$CIRCLE_BRANCH" != "master"  && "$CIRCLE_BRANCH" != release*  ]]; then
 
@@ -12,9 +17,14 @@ if [[ "$CIRCLE_BRANCH" != "master"  && "$CIRCLE_BRANCH" != release*  ]]; then
   # Ensure presensce of upstream remote
   git ls-remote --exit-code --quiet upstream 2>/dev/null || git remote add upstream https://github.com/freedomofpress/securedrop.git
 
+  # Determine target branch via API
+  #
+  # (We are togglin' between two upstream containers here - the constant is both have python3)
+  target_branch="$(curl -s ${GITHUB_PR_URL} | python3 -c 'import sys, json; print(json.load(sys.stdin)["base"]["ref"])')"
+
   # Fetch and rebase onto the latest in develop
-  git fetch upstream develop
-  git rebase upstream/develop
+  git fetch upstream "${target_branch}"
+  git rebase "upstream/${target_branch}"
 
   # Print out the current head for debugging potential CI issues
   git rev-parse HEAD