Add API endpoint to request contents of multiple files simultaniously (#34139)

Adds an API POST endpoint under `/repos/{owner}/{repo}/file-contents`
which receives a list of paths and returns a list of the contents of
these files.

This API endpoint will be helpful for applications like headless CMS
(reference: https://github.com/sveltia/sveltia-cms/issues/198) which
need to retrieve a large number of files by reducing the amount of
needed API calls.

Close #33495

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/repository/files/update.go

@@ -22,6 +22,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/routers/api/v1/utils"
 	asymkey_service "code.gitea.io/gitea/services/asymkey"
 	pull_service "code.gitea.io/gitea/services/pull"
 )
@@ -296,7 +297,9 @@ func ChangeRepoFiles(ctx context.Context, repo *repo_model.Repository, doer *use
 		return nil, err
 	}
 
-	filesResponse, err := GetFilesResponseFromCommit(ctx, repo, commit, opts.NewBranch, treePaths)
+	// FIXME: this call seems not right, why it needs to read the file content again
+	// FIXME: why it uses the NewBranch as "ref", it should use the commit ID because the response is only for this commit
+	filesResponse, err := GetFilesResponseFromCommit(ctx, repo, utils.NewRefCommit(git.RefNameFromBranch(opts.NewBranch), commit), treePaths)
 	if err != nil {
 		return nil, err
 	}