C++: Improve completion assist

Avoid inserting characters that are already there.
e.g.
int testFly() {}
te|Fly()
When triggers a completion, the code will changed to:
testFly()|
But in before, the code will changed to:
testFly()|Fly()

Notice:  Character of '|' show the cursor position.

Change-Id: I6c111e30356b2549aebc6fbea05608e70d6ab41f
Merge-request: 398
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>