Update `GitStore.on_buffer_store_event` so that, when a
`BufferStoreEvent::BufferChangedFilePath` event is received, we check if
there's any diff state for the buffer and, if so, update it according to
the new file path, in case the file exists in the repository.

Closes #40499

Release Notes:

- Fixed issue with git diff tracking when updating a buffer's file from
an untracked to a tracked file