Bitbucket PR import: case when we already have all changesets

This is the simplest case of #169 (closed). In case hg pull_force_topic does not pull anything (now corresponding to exit code 1), and the wanted changeset is visible, we can just adapt by reading the branch/topic information on it, which gives us all needed info to create the MR.

The more complex case in which the target changeset is known but is obsolete will maybe be handled entirely in hg pull_force_topic: for now the hg log call will refuse it, and we couldn't do anything with it on the Rails side, because the corresponding Git commit is not there or has become unreachable.