feat(api): add last_commit_when to contents response (#7418)

- Add a new field `last_commit_when` to the `ContentResponse` type, which is populated with the last commit's commiter date. This can be used to determine when the last edit of the content was.
- This field is compatible with what Gitea will likely add, https://github.com/go-gitea/gitea/pull/32921. There's no field for this information in the Github API, so no way to be compatible with that (this API endpoint is otherwise fully compatible with Github's API).
- Ref: gitnex/GitNex#1225
- Integration test adjusted. The API tests cannot test the actual output, as `testify` tries to 'deep equal' the `time.Time` structs which will differ due how the `time.Time` struct is created. Unit tests still verify the output.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7418
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

services/repository/files/content.go

@@ -178,12 +178,13 @@ func GetContents(ctx context.Context, repo *repo_model.Repository, treePath, ref
 
 	// All content types have these fields in populated
 	contentsResponse := &api.ContentsResponse{
-		Name:          entry.Name(),
-		Path:          treePath,
-		SHA:           entry.ID.String(),
-		LastCommitSHA: lastCommit.ID.String(),
-		Size:          entry.Size(),
-		URL:           &selfURLString,
+		Name:           entry.Name(),
+		Path:           treePath,
+		SHA:            entry.ID.String(),
+		LastCommitSHA:  lastCommit.ID.String(),
+		LastCommitWhen: lastCommit.Committer.When,
+		Size:           entry.Size(),
+		URL:            &selfURLString,
 		Links: &api.FileLinksResponse{
 			Self: &selfURLString,
 		},