Show only commits into certain branch on Sublime Merge

As of build 1070, you can do this by Alt-clicking the eye icon on the branch name in the location bar:

Example:

Build 1107 has a pretty convenient solution to the OP's problem. (I'm not sure if that's when they started including it though).

Simply right-click on the commit that the branch you want to isolate is at and choose to hide all branches except the relevant one.