Single completion with matching buffer

[tkruse]

Currently the CandidateListCompletionHandler does nothing and returns false if the buffer is equal to the candidate. This is flawed in 2 ways.

First, the question should not be whether the full buffer is equal to the candidate, but only the respective region that would be changed.

Second, I think even when that region is the same, the completion handler should move the cursor to the position behind the completion value. Such that hitting tab inside an argument, when the only candidate is the existing argument, will still move your cursor to the end of the argument.