You misunderstand. It has nothing to do with branching. (And, as @MattDMo writes, you are attacking the problem from the wrong end.) Humans cannot digest cellulose because they do not have the enzymes needed to do so. These are called cellulases, which you can read about here. The problem seems to be one, not only of breaking the β(1→4) glucose linkage, but disrupting the higher-order structure formed by the interaction of individual cellulose chains.
Ruminants, that need to digest the cellulose in grass, also cannot produce cellulases, but they host bacteria to do the job for them in their rumen. Presumably if we humans needed to live on grass our stomach evolution would have followed a similar path.