backout: reverse changeset belongs on current branch

Backing out a changeset that is before a named branch branchpoint was
making the reverse changeset the tip of the old branch, which is wrong
and very confusing. So instead, we put it on the current named branch.

mercurial/commands.py

@@ -227,7 +227,10 b' def backout(ui, repo, node=None, rev=Non'
             raise util.Abort(_('cannot use --parent on non-merge changeset'))
         parent = p1
 
+    # the backout should appear on the same branch
+    branch = repo.dirstate.branch()
     hg.clean(repo, node, show_stats=False)
+    repo.dirstate.setbranch(branch)
     revert_opts = opts.copy()
     revert_opts['date'] = None
     revert_opts['all'] = True

tests/test-backout.out

@@ -74,7 +74,7 b' marked working directory as branch branc'
 adding file2
 removing file1
 created new head
-changeset 3:f1c642b1d8e5 backs out changeset 1:bf1602f437f3
+changeset 3:d4e8f6db59fb backs out changeset 1:bf1602f437f3
 the backout changeset is a new head - do not forget to merge
 (use "backout --merge" if you want to auto-merge)
 % on branch2 with branch1 not merged, so file1 should still exist:
@@ -85,10 +85,11 b' C file2'
 % on branch2 with branch1 merged, so file1 should be gone:
 0 files updated, 0 files merged, 1 files removed, 0 files unresolved
 (branch merge, don't forget to commit)
-21d4dc6f9a41 (branch2) tip
+22149cdde76d (branch2) tip
 C default
 C file2
 % on branch1, so no file1 and file2:
-0 files updated, 0 files merged, 1 files removed, 0 files unresolved
-f1c642b1d8e5 (branch1)
+1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+bf1602f437f3 (branch1)
 C default
+C file1