run ci for each commit

GitLab doesn't seem to have built-in support for this because of course
it doesn't.

To do this, we move the job scripts to a different file to make it
possible to share code between job scripts.

bin/job

@@ -24,8 +24,9 @@ job_pages() (
 bail() (
     set -euo pipefail
 
+    git show --shortstat
     echo
-    echo "Job failed"
+    echo "Failure caused by the above commit"
     exit 1
 )
 
@@ -47,7 +48,20 @@ run() (
 
     cd "$(git rev-parse --show-toplevel)"
 
-    job_"$job" || bail
+    if [[ -z "${CI_MERGE_REQUEST_DIFF_BASE_SHA+x}" ]]; then
+        echo "Running against latest commit only..."
+
+        job_"$job" || bail
+    else
+        echo "Running against all commits since this branch's base..."
+
+        readarray -t commits < \
+            <(git rev-list --reverse "$CI_MERGE_REQUEST_DIFF_BASE_SHA..HEAD")
+        for commit in "${commits[@]}"; do
+            git checkout "$commit"
+            job_"$job" || bail
+        done
+    fi
 )
 
 run "$@"