prev: jump to parent's successor if parent is obsolete and topic is involved `hg stack` shows a linear chain of commits even when they are not linear chain and some changesets are obsoleted and have successors as separate head. This is very nice to have a post evolution view of the stack. However when `hg prev` is run on an unstable changeset whose parent is obsolete and does not topic but parents successor has the topic, it shows no parent on the topic. This patch makes `hg prev` update to parent's successor and make things follow the stack order.

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#require test-repo

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  $ checkflake8() {

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  >   if ! (which flake8 > /dev/null); then

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  >     echo skipped: missing tool: flake8;

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  >     exit 80;

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  >   fi;

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  > };

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  $ checkflake8

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Copied from Mercurial core (60ee2593a270)

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  $ cd "`dirname "$TESTDIR"`"

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run flake8 if it exists; if it doesn't, then just skip

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  $ hg files -0 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \

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  > | xargs -0 flake8