feat: adds option to force update new branch in contents routes (#35592)

Allows users to specify a "force" option in API /contents routes when
modifying files in a new branch. When "force" is true, and the branch
already exists, a force push will occur provided the branch does not
have a branch protection rule that disables force pushing.

This is useful as a way to manage a branch remotely through only the
API. For example in an automated release tool you can pull commits,
analyze, and update a release PR branch all remotely without needing to
clone or perform any local git operations.

Resolve #35538

---------

Co-authored-by: Rob Gonnella <rob.gonnella@papayapay.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/repo/file.go

@@ -355,6 +355,7 @@ func ReqChangeRepoFileOptionsAndCheck(ctx *context.APIContext) {
 		Message:   commonOpts.Message,
 		OldBranch: commonOpts.BranchName,
 		NewBranch: commonOpts.NewBranchName,
+		ForcePush: commonOpts.ForcePush,
 		Committer: &files_service.IdentityOptions{
 			GitUserName:  commonOpts.Committer.Name,
 			GitUserEmail: commonOpts.Committer.Email,
@@ -591,6 +592,11 @@ func UpdateFile(ctx *context.APIContext) {
 }
 
 func handleChangeRepoFilesError(ctx *context.APIContext, err error) {
+	if git.IsErrPushRejected(err) {
+		err := err.(*git.ErrPushRejected)
+		ctx.APIError(http.StatusForbidden, err.Message)
+		return
+	}
 	if files_service.IsErrUserCannotCommit(err) || pull_service.IsErrFilePathProtected(err) {
 		ctx.APIError(http.StatusForbidden, err)
 		return