The incremental diff functionality that was added in Sublime build 3200 can tell you what lines are different from the
index in the git repository (depending on the value of the
git_diff_target setting) and that works without having to install anything extra (
Sublime Merge, etc) though it does require that you use a 64-bit version of Sublime.
It only diffs against those two items, though. Based on your question it’s unclear if that would do what you want or if you literally want it to always show you the differences between the current file and the
master branch (i.e. regardless of what the current branch is, state of the repository in general, etc).
If you need that, then you could install GitGutter; in that package the
compare_against setting can be any tag, branch or commit hash.