Annotate branch existence fields as calling gitaly

In general they will not, since their values will most likely be cached
in redis, but they will call gitaly when the cache is cold.

app/graphql/types/merge_request_type.rb

@@ -81,8 +81,14 @@ module Types
           description: 'Default merge commit message of the merge request'
     field :merge_ongoing, GraphQL::BOOLEAN_TYPE, method: :merge_ongoing?, null: false,
           description: 'Indicates if a merge is currently occurring'
-    field :source_branch_exists, GraphQL::BOOLEAN_TYPE, method: :source_branch_exists?, null: false,
+    field :source_branch_exists, GraphQL::BOOLEAN_TYPE,
+          null: false, calls_gitaly: true,
+          method: :source_branch_exists?,
           description: 'Indicates if the source branch of the merge request exists'
+    field :target_branch_exists, GraphQL::BOOLEAN_TYPE,
+          null: false, calls_gitaly: true,
+          method: :target_branch_exists?,
+          description: 'Indicates if the target branch of the merge request exists'
     field :mergeable_discussions_state, GraphQL::BOOLEAN_TYPE, null: true,
           description: 'Indicates if all discussions in the merge request have been resolved, allowing the merge request to be merged'
     field :web_url, GraphQL::STRING_TYPE, null: true,

spec/graphql/types/merge_request_type_spec.rb

@@ -19,7 +19,8 @@ describe GitlabSchema.types['MergeRequest'] do
       force_remove_source_branch merge_status in_progress_merge_commit_sha
       merge_error allow_collaboration should_be_rebased rebase_commit_sha
       rebase_in_progress merge_commit_message default_merge_commit_message
-      merge_ongoing source_branch_exists mergeable_discussions_state web_url
+      merge_ongoing mergeable_discussions_state web_url
+      source_branch_exists target_branch_exists
       upvotes downvotes head_pipeline pipelines task_completion_status
       milestone assignees participants subscribed labels discussion_locked time_estimate
       total_time_spent reference