histedit: preserve active branch while histediting

The branch information was properly preserved in the changeset, but the
"active" branch of the working copy could be lost (the branch of the base
being used).

Histedit used to behave properly in this regard but the case was not tested
and regressed 4 years ago in ab2362e1672e.

hgext/histedit.py

@@ -487,6 +487,7 b' class histeditaction(object):'
         repo.ui.pushbuffer(error=True, labeled=True)
         hg.update(repo, self.state.parentctxnode, quietempty=True)
         stats = applychanges(repo.ui, repo, rulectx, {})
+        repo.dirstate.setbranch(rulectx.branch())
         if stats and stats[3] > 0:
             buf = repo.ui.popbuffer()
             repo.ui.write(*buf)

tests/test-histedit-commute.t

@@ -454,3 +454,36 b" Now, let's try to fold the second commit"
   rename to another-dir/renamed-file
 
   $ cd ..
+
+Test that branches are preserved and stays active
+-------------------------------------------------
+
+  $ hg init repo-with-branch
+  $ cd repo-with-branch
+  $ echo a > a
+  $ hg add a
+  $ hg commit -m A
+  $ hg branch foo
+  marked working directory as branch foo
+  (branches are permanent and global, did you want a bookmark?)
+  $ echo a > b
+  $ hg add b
+  $ hg commit -m foo-B
+  $ echo a > c
+  $ hg add c
+  $ hg commit -m foo-C
+
+  $ hg branch
+  foo
+  $ echo "pick efefa76d6dc3 2 foo-C" >> cmd
+  $ echo "pick 7336e7550422 1 foo-B" >> cmd
+
+  $ HGEDITOR=cat hg histedit -r ".^" --commands cmd --quiet
+  $ hg log --template '{rev} {branch}\n'
+  2 foo
+  1 foo
+  0 default
+  $ hg branch
+  foo
+
+  $ cd ..