extra: add git pre-push hook for triggering Docker Hub builds

This isn't that good either as the resulting CI run will end up running before
the new image builds are finished. But it's better than a post-commit hook
which will almost certainly trigger a build without the desired new changes
present in the repo.

extra/git-pre-push.sh

@@ -0,0 +1,41 @@
+#!/bin/sh -e
+
+# Pre-push hook for triggering bash-completion Docker Hub test image
+# builds at https://hub.docker.com/r/vskytta/bash-completion/
+#
+# To enable: ln -s ../../extra/git-pre-push.sh .git/hooks/pre-push
+#
+# The bash-completion.docker-hub-trigger-url config option must be set to
+# the full Docker Hub build trigger URL to hit.
+
+url=$(git config bash-completion.docker-hub-trigger-url) || exit 0
+
+branch=master
+files="completions/Makefile\.am|test/requirements\.txt"
+
+trigger=false
+z40=0000000000000000000000000000000000000000
+
+while read local_ref local_sha remote_ref remote_sha; do
+    case $remote_ref in */$branch) ;; *) continue ;; esac
+    [ $local_sha != $z40 ] || continue  # delete not handled (yet?)
+    if [ $remote_sha = $z40 ]; then
+        list_files="git ls-tree -r --name-only $local_sha"
+    else
+        list_files="git diff --name-only $remote_sha..$local_sha"
+    fi
+    ! $list_files | grep -qEx $files || { trigger=true; break; }
+done
+
+if $trigger; then
+    cat <<EOF | at -M now
+        sleep 15
+        curl \
+             --silent --show-error \
+             --max-time 30 \
+             --header Content-Type:application/json \
+             --data '{"build":true}' \
+             $url 2>&1 \
+        | logger -e --tag bash-completion-pre-push
+EOF
+fi