Improve one common case for multiline diffing

[vors]

Addresses the following case:

def x():
    pass

def y():
    pass

def z():
    pass

def x():
    pass

def z():
    pass

Previously

Now

Implementation

We piggy-back on the existing diff_main opt_checklines bool flag to make the common prefix and suffix heuristics respect this flag. Concretely they will only cut away the prefix/suffix that ends/starts with \n.

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This change is