Add MergeRequest.diffStatsSummary.fileCount to graphql API

This allows the number of files changed in a MR to be counted, using
queries such as:

```graphql
query {
  project(fullPath: "group/project") {
    mergeRequest(iid: "1") {
      diffStatsSummary { fileCount }
    }
  }
}
```

app/graphql/types/diff_stats_summary_type.rb

@@ -14,6 +14,8 @@ module Types
           description: 'Number of lines deleted'
     field :changes, GraphQL::INT_TYPE, null: false,
           description: 'Number of lines changed'
+    field :file_count, GraphQL::INT_TYPE, null: false,
+          description: 'Number of files changed'
 
     def changes
       object[:additions] + object[:deletions]

app/graphql/types/merge_request_type.rb

@@ -153,11 +153,11 @@ module Types
     end
 
     def diff_stats_summary
-      nil_stats = { additions: 0, deletions: 0 }
+      nil_stats = { additions: 0, deletions: 0, file_count: 0 }
       return nil_stats unless object.diff_stats.present?
 
       object.diff_stats.each_with_object(nil_stats) do |status, hash|
-        hash.merge!(additions: status.additions, deletions: status.deletions) { |_, x, y| x + y }
+        hash.merge!(additions: status.additions, deletions: status.deletions, file_count: 1) { |_, x, y| x + y }
       end
     end
   end

changelogs/unreleased/ajk-gql-diff-stats-file-count.yml

+---
+title: Add MergeRequest.diffStatsSummary.fileCount to graphql API
+merge_request: 35685
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -3164,6 +3164,11 @@ type DiffStatsSummary {
   Number of lines deleted
   """
   deletions: Int!
+
+  """
+  Number of files changed
+  """
+  fileCount: Int!
 }
 
 type Discussion implements ResolvableInterface {

doc/api/graphql/reference/gitlab_schema.json

@@ -8778,6 +8778,24 @@
               "description": "Number of lines deleted",
               "args": [
 
+              ],
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "Int",
+                  "ofType": null
+                }
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
+            {
+              "name": "fileCount",
+              "description": "Number of files changed",
+              "args": [
+
               ],
               "type": {
                 "kind": "NON_NULL",

doc/api/graphql/reference/index.md

@@ -545,6 +545,7 @@ Aggregated summary of changes
 | `additions` | Int! | Number of lines added |
 | `changes` | Int! | Number of lines changed |
 | `deletions` | Int! | Number of lines deleted |
+| `fileCount` | Int! | Number of files changed |
 
 ## Discussion
 

spec/requests/api/graphql/project/merge_request_spec.rb

@@ -55,7 +55,12 @@ RSpec.describe 'getting merge request information nested in a project' do
 
     expect(merge_request_graphql_data).to include(
       'diffStats' => all(a_hash_including('path' => String, 'additions' => be_natural, 'deletions' => be_natural)),
-      'diffStatsSummary' => a_hash_including('additions' => be_natural, 'deletions' => be_natural, 'changes' => be_natural)
+      'diffStatsSummary' => a_hash_including(
+        'fileCount' => merge_request.diff_stats.count,
+        'additions' => be_natural,
+        'deletions' => be_natural,
+        'changes' => be_natural
+      )
     )
 
     # diff_stats is consistent with summary