bugfix the compare url with the changes on the dependency between starting revision and candidate

The core procedure of uprev builds a "compare" url to report the developers which changes happen in the dependency between the current revision (aka A) used in the target project to the proposed candidate to uprev (aka B).

Due to the modification to check if there is a proposal in progress (if a previous uprev execution produced a merge request, where the developers may have contributed) this url didn't compare the complete range. As it continues a previous attempt, the proposed candidate is newer (aka C) and the url, instead of compare A...C, compares B...C. Internally this is because the core process uses as current revision (A) what's in the file to modify, when as a continuation it sees the previous proposal (B).

With this set of changes, it tries a dry uprev of what's in the default branch to know which is really the current revision (A). Then, when it proceeds to create a candidate, this current revisions (A) is injected to the object in charge of the uprev to make sure it is correctly used to build the diff url from A to C.