branches: correctly show inactive multiheaded branches Issue being fixed here: `hg branches` incorrectly renders inactive multiheaded branches as active if they have closed heads. Example: ``` $ hg log --template '{rev}:{node|short} "{desc}" ({branch}) [parents: {parents}]\n' 4:2e2fa7af8357 "merge" (default) [parents: 0:c94e548c8c7d 3:7be622ae5832 ] 3:7be622ae5832 "2" (somebranch) [parents: 1:81c1d9458987 ] 2:be82cf30409c "close" (somebranch) [parents: ] 1:81c1d9458987 "1" (somebranch) [parents: ] 0:c94e548c8c7d "initial" (default) [parents: ] $ hg branches default 4:2e2fa7af8357 somebranch 3:7be622ae5832 ``` Branch `somebranch` have two heads, the 1st one being closed (rev 2) and the other one being merged into default (rev 3). This branch should be shown as inactive one. This happens because we intersect branch heads with repo heads to check branch activity. In this case intersection in a set with one node (rev 2). This head is closed but the branch is marked as active nevertheless. Fix is to check branch activity by intersecting only open heads set. Fixed output: ``` $ hg branches default 4:2e2fa7af8357 somebranch 3:7be622ae5832 (inactive) ``` Relevant tests for multihead branches added to test-branches suite. Implentation note about adding `iteropen` method: At first I have tried to modify `iterbranches` is such a way that it would filter out closed heads itself. For example it could have `closed=False` parameter. But in this case we would have to filter closed tips as well. Reasoning in terms of `hg branches` we actually are not allowed to do this. Also, we need to do heads filtering only if tip is not closed itself. But if it is - we are ok to skip filtering, because branch is already known to be inactive. So we can't implement heads filtering in `iterbranches` in elegant way, because we will end up with something like `closed_heads=False` or even `closed_heads_is_tip_is_open`. Finally I decided to move this logic to the `branches` function, adding `iteropen` helper method. Differential Revision: https://phab.mercurial-scm.org/D583