search: explicitly dissallow multi branch structural search
Created by: keegancsmith
Structural search will only search indexed commits. Previously we did not need to explicitly exclude multi branch search since indexed search only supported searching HEAD. However, now indexed search could search other revs.
We still need to update structural search to support non-HEAD search. For now we dissallow it to prevent bugs.
Part of https://github.com/sourcegraph/sourcegraph/issues/11906 and #6728