topic: add a 'topicidx()' to context This will be useful for the `t0` logic in `hg prev`

prev: retrieve the 'no-topic' result is a clear and reusable option We'll use it more, so let us make it pretty first.

prev: add '_getcurrenttopic' helper and use it We'll add more spot where this information is used.

prev: explicitly use 'p1' as a variable name This make the code clearer.

topic: rename the internal topic option we'll use it more widely so I give it a more generic name. It is an internal option so not wories regarding compatibility.

prev: extract the code computing the destination This make the handling of the result simpler. The test agree as one return code is fixed.

prev: issue a proper error when reaching repository root (and adds the associated test)

prev: stop creating a repository at the root of 'test-repo.t' We run the test in a subdirectory now, this usually avoid various kind of future issue.

prev: extract the function performing the update That code is fairly independant so let us isolate it to simplify the rest of the code.

prev: simplify the bookmark preserving logic We can use 'None' as a special value and have a simpler code.

prev: rename 'p' to 'target' This makes the code clearer.

topics: check for topic on commit before a user enters message We have a enforce-topic cofig which can forbid user to commit without a topic on it. We used to check topic on a commit after the user enters message, but we should fail early.