destutil: document various failure cases We document what various conditional branch mean and clarify that they are exclusive (since they all end up in with exception raised).

destutil: consistently retrieve 'p1' and 'branch' We already read p1 from the dirstate so let's read the branch from it too.

merge: give priority to "not at head" failures for bare 'hg merge' We refuse to pick a destination for a bare 'hg merge' if the working copy is not at head. This is meant to prevent strange merge from user who forget to update. (Moreover, such merge does not reduce actually the number of heads) However, we were doing that as the last possible failure type. So user were recommended to merge with an explicit head (from this bad location) if the branch had too many heads. We now make "not on branch heads" class of failure the first things to check and fail on. The one test that change was actually trying to check for these failure (and did not). The new test output is correct.