Support long diff conflict markers
git can be configured to use more than 7 characters for conflict markers, and jj automatically uses longer conflict markers when the text contains any char sequence that could be confused with conflict markers. Ensure that we only point at markers that are consistent with the start marker's length.
Ensure that we only consider char sequences at the beginning of a line as a diff marker.
Fix https://github.com/rust-lang/rust/issues/150352.
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