circleci: merge (and reattempt) PR before testing

.circleci/config.yml

@@ -6,6 +6,17 @@ common: &common
   working_directory: ~/repo
   steps:
     - checkout
+    - run:
+        name: merge pull request base
+        command: ./.circleci/merge_pr.sh
+    - run:
+        name: merge pull request base (2nd try)
+        command: ./.circleci/merge_pr.sh
+        when: on_fail
+    - run:
+        name: merge pull request base (3nd try)
+        command: ./.circleci/merge_pr.sh
+        when: on_fail
     - run:
         name: merge pull request base
         command: |
@@ -33,6 +44,7 @@ common: &common
         command: ~/.local/bin/tox -r
     - save_cache:
         paths:
+          - .hypothesis
           - .tox
           - ~/.cache/pip
           - ~/.local

.circleci/merge_pr.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+if [[ -n "${CIRCLE_PR_NUMBER}" ]]; then
+  PR_INFO_URL=https://api.github.com/repos/$CIRCLE_PROJECT_USERNAME/$CIRCLE_PROJECT_REPONAME/pulls/$CIRCLE_PR_NUMBER
+  PR_BASE_BRANCH=$(curl -L "$PR_INFO_URL" | python -c 'import json, sys; obj = json.load(sys.stdin); sys.stdout.write(obj["base"]["ref"])')
+  git fetch origin +"$PR_BASE_BRANCH":circleci/pr-base
+  # We need these config values or git complains when creating the
+  # merge commit
+  git config --global user.name "Circle CI"
+  git config --global user.email "circleci@example.com"
+  git merge --no-edit circleci/pr-base
+fi