Fix commit behvaior where items use default commit chars as their special filter char

[genlu]

We never handled this correctly, but the bug wasn't exposed until operator completion was added (where '+', '-', etc. are used as filter chars)

Fix #70732

[ToddGrun]

foreach (var kvp in excludedCommitCharactersFromList)

nit: would be nice if there were an AddRange on MultiDictionary. Would simplify this code and it's implementation would be slightly more performant as it would only need to look up each key once

[genlu]

I think AddRange is usually associated with list like types in BCL. Maybe we could add a Union for MultiDictionary? I can give this a try in a follow up PR since it'd require change in compiler layer. Also import compeltion uses this type extensively so I'd like to take that into consideration too.

[ToddGrun]

Sounds good, I just saw that OrderedMultiDictionary has an AddRange, so I thought the name could match that, but Union is fine by me too.

[ToddGrun]

Out of curiosity, this method must get called on the same session multiple times with different completionList.ItemsList values, right? Just trying to understand if there is a relation between previous values in the ExcludedCommitCharactersMap property and the items returned from GetExcludedCommitCharacters.

[genlu]

Yes, currently it could be called twice for a given session to support import completion, as we might delay those unimported item to a second batch w/o showing them until the list refreshed, or if import completion is disabled, it can be explicitly triggered by using the expander button during the same session. Which means the values in the map from before vs after are from two disjoint sets of completion items (unimported item vs everything else).

[ToddGrun]