branching: merge with stable

prev: allow to update to 't0' from 't1' When on the first changeset of a topic, one can move back one changeset to end up on t0. The new working copy parent to not belong to the topic, but the topic is still active. The next commit will create a new root to the active topic. You can get the same result using `hg up t0`

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.