Expose the target commit ID through the tag API

This is useful for annotated tags, where the deferenced target is not the same as the tag object. At the moment there is no way to differentiate the two through the tag API.

This change adds a "target" property and leaves the existing "commit" property alone so that existing behaviour is not altered.

changelogs/unreleased/feature-add_target_to_tags.yml

+---
+title: Expose the target commit ID through the tag API
+merge_request:
+author:
+type: added

doc/api/tags.md

@@ -42,6 +42,7 @@ Parameters:
       "description": "Amazing release. Wow"
     },
     "name": "v1.0.0",
+    "target": "2695effb5807a22ff3d138d593fd856244e155e7",
     "message": null
   }
 ]
@@ -73,6 +74,7 @@ Example Response:
 {
   "name": "v5.0.0",
   "message": null,
+  "target": "60a8ff033665e1207714d6670fcd7b65304ec02f",
   "commit": {
     "id": "60a8ff033665e1207714d6670fcd7b65304ec02f",
     "short_id": "60a8ff03",
@@ -132,12 +134,16 @@ Parameters:
     "description": "Amazing release. Wow"
   },
   "name": "v1.0.0",
+  "target: "2695effb5807a22ff3d138d593fd856244e155e7",
   "message": null
 }
 ```
 The message will be `null` when creating a lightweight tag otherwise
 it will contain the annotation.
 
+The target will contain the tag objects ID when creating annotated tags,
+otherwise it will contain the commit ID when creating lightweight tags.
+
 In case of an error,
 status code `405` with an explaining error message is returned.
 

lib/api/entities.rb

@@ -928,7 +928,7 @@ module API
     end
 
     class Tag < Grape::Entity
-      expose :name, :message
+      expose :name, :message, :target
 
       expose :commit, using: Entities::Commit do |repo_tag, options|
         options[:project].repository.commit(repo_tag.dereferenced_target)

spec/fixtures/api/schemas/public_api/v4/tag.json

@@ -10,6 +10,7 @@
     "name": { "type": "string" },
     "message": { "type": ["string", "null"] },
     "commit": { "$ref": "commit/basic.json" },
+    "target": { "type": "string" },
     "release": {
       "oneOf": [
         { "type": "null" },