Fix strict Argument checking broken with relative completion offset

[tkruse]

I found a small regression in ArgumentCompleter when one of the involved completers returns a partial completion (meaning one with an offset. I believe this was introduced here:

2de5902

-            if (subCandidates.size() == 0) {
+            if (!subCandidates.contains(arg)) {

[tkruse]

I found another problem if the candidate has a delimiter appended. Fixed that, but the code looks bloated. Not sure if anyone can find a more elegant solution.