Add "returning" option for merge output ref

Setting the `returning` param to `unmerged` allows using the original
commit as the resource output when setting the `merge` param to `true`.

Resolves concourse#253

README.md

@@ -214,6 +214,10 @@ and the `rebase` parameter is not provided, the push will fail.
   attempt to merge remote to local before pushing. Only one of `merge` or
   `rebase` can be provided, but not both.
 
+* `returning`: *Optional.* When passing the `merge` flag, specify whether the
+  merge commit or the original, unmerged commit should be passed as the output
+  ref. Options are `merged` and `unmerged`. Defaults to `merged`.
+
 * `tag`: *Optional.* If this is set then HEAD will be tagged. The value should be
   a path to a file containing the name of the tag.
 

assets/out

@@ -35,6 +35,7 @@ tag=$(jq -r '.params.tag // ""' < $payload)
 tag_prefix=$(jq -r '.params.tag_prefix // ""' < $payload)
 rebase=$(jq -r '.params.rebase // false' < $payload)
 merge=$(jq -r '.params.merge // false' < $payload)
+returning=$(jq -r '.params.returning // "merged"' < $payload)
 force=$(jq -r '.params.force // false' < $payload)
 only_tag=$(jq -r '.params.only_tag // false' < $payload)
 annotation_file=$(jq -r '.params.annotate // ""' < $payload)
@@ -196,7 +197,13 @@ else
   add_and_push_notes
 fi
 
+if [ "$merge" = "true" ] && [ "$returning" = "unmerged" ]; then
+  version_ref="$(echo "$commit_to_push" | jq -R .)"
+else
+  version_ref="$(git rev-parse HEAD | jq -R .)"
+fi
+
 jq -n "{
-  version: {ref: $(git rev-parse HEAD | jq -R .)},
+  version: {ref: $version_ref},
   metadata: $(git_metadata)
 }" >&3

test/helpers.sh

@@ -790,6 +790,20 @@ put_uri_with_merge() {
   }" | ${resource_dir}/out "$2" | tee /dev/stderr
 }
 
+put_uri_with_merge_returning_unmerged() {
+  jq -n "{
+    source: {
+      uri: $(echo $1 | jq -R .),
+      branch: \"master\"
+    },
+    params: {
+      repository: $(echo $3 | jq -R .),
+      merge: true,
+      returning: unmerged
+    }
+  }" | ${resource_dir}/out "$2" | tee /dev/stderr
+}
+
 put_uri_with_merge_and_rebase() {
   jq -n "{
     source: {

test/put.sh

@@ -274,6 +274,43 @@ it_can_put_to_url_with_merge_commit() {
   test $latest_merge_ref = $merged_ref
 }
 
+it_chooses_the_unmerged_commit_ref() {
+  local repo1=$(init_repo)
+
+  local src=$(mktemp -d $TMPDIR/put-src.XXXXXX)
+  local repo2=$src/repo
+  git clone $repo1 $repo2
+
+  # make a commit that will require rebasing
+  local baseref=$(make_commit_to_file $repo1 some-other-file)
+
+  local unmerged_ref=$(make_commit $repo2)
+
+  # cannot push to repo while it's checked out to a branch
+  git -C $repo1 checkout refs/heads/master
+
+  local response=$(mktemp $TMPDIR/rebased-response.XXXXXX)
+
+  put_uri_with_merge_returning_unmerged $repo1 $src repo > $response
+
+  local merged_ref=$(git -C $repo2 rev-parse HEAD)
+
+  jq -e "
+    .version == {ref: $(echo $unmerged_ref | jq -R .)}
+  " < $response
+
+  # switch back to master
+  git -C $repo1 checkout master
+
+  test -e $repo1/some-file
+
+  test "$(git -C $repo1 rev-parse HEAD)" = $merged_ref
+
+  local latest_merge_ref=$(git -C $repo1 log -n 1 --merges --pretty=format:"%H")
+
+  test $latest_merge_ref = $merged_ref
+}
+
 it_will_fail_put_if_merge_and_rebase_are_set() {
   local repo1=$(init_repo)