comparison: "scroll" the `-v` output to the first diff

[blueyed]

E.g. with:

E   AssertionError: assert ['version', '...version_info'] == ['version', '...version', ...]
E     Right contains 3 more items, first extra item: ' '
E     Full diff:
E       [
E        'version',
E        'version_info',
E        'sys.version',
E        'sys.version_info',...
E
E     ...Full output truncated (5 lines hidden), use '-vv' to show

It should include / start at the first line that starts with "+" / "-".

Originally posted by @blueyed in #5924 (comment)