Clean up CommitMatch string types

Created by: camdencheek

This pushes the MatchedString type up the stack so that CommitMatch uses MatchedString instead of HighlightedString. This is useful because MatchedString retains multiline information. Additionally, this moves the conversion code from MatchedString to HighlightedString into the internal/search/result package, which makes sense because both of those types are defined there.

There should be no change in behavior here. It just pushes the conversion to a HighlightedString up to the GraphQL layer, since that's where the "highlights are only one line" constraint actually exists. The change is split up into three smaller commits.