Fix return type for Reference.isHead (typed as boolean, but returns a number)

[dsabanin]

Both API docs and TypeScript typings say that Reference.isHead returns boolean, while in fact it's just a wrapper around Branch.isHead that returns 0 for false, 1 for true.

This change makes the code do what published API already says it does. Branch.isHead is correctly typed in API as returning number.

[rcjsuen]

I wonder if it might make more sense to change Branch.isHead to return true or false so that it's more intuitive instead of letting it return 0 or 1...

[dsabanin]

I agree, but I'm not sure what the policy on API changes is. The Branch.isHead is behaving according to the published API, which has it as returning number.

[rcjsuen]

@dsabanin For better or worse, we've changed API before. ;) Many functions have changed from synchronous to being asynchronous.