commit: check branch change on to-be-committed changeset

This makes a difference when a branch name is passed via the `extra` argument,
which will be used as the branch name for the created changeset. The empty
commit check should use that branch name instead of whatever was set on the
working directory.

Besides fixing the bug for which a test case was extended, it enables to remove
a workaround in the rebase extension, which will be done in a follow-up patch
(D8725).

Differential Revision: https://phab.mercurial-scm.org/D8724

mercurial/localrepo.py

@@ -2996,7 +2996,7 b' class localrepository(object):'
 
             # internal config: ui.allowemptycommit
             allowemptycommit = (
-                wctx.branch() != wctx.p1().branch()
+                cctx.branch() != cctx.p1().branch()
                 or extra.get(b'close')
                 or merge
                 or cctx.files()

tests/test-rebase-emptycommit.t

@@ -129,7 +129,16 b' Merge and its ancestors all become empty'
   > B
   > EOS
 
-  $ hg rebase -r '(A::)-(B::)-A' -d H
+Previously, there was a bug where the empty commit check compared the parent
+branch name with the wdir branch name instead of the actual branch name (which
+should stay unchanged if --keepbranches is passed), and erroneously assumed
+that an otherwise empty changeset should be created because of the incorrectly
+assumed branch name change.
+
+  $ hg update H -q
+  $ hg branch foo -q
+
+  $ hg rebase -r '(A::)-(B::)-A' -d H --keepbranches
   rebasing 2:dc0947a82db8 "C" (BOOK-C)
   note: not rebasing 2:dc0947a82db8 "C" (BOOK-C), its destination already has all its changes
   rebasing 3:b18e25de2cf5 "D" (BOOK-D)
@@ -137,6 +146,7 b' Merge and its ancestors all become empty'
   rebasing 4:86a1f6686812 "E" (BOOK-E E)
   note: not rebasing 4:86a1f6686812 "E" (BOOK-E E), its destination already has all its changes
   saved backup bundle to $TESTTMP/merge1/.hg/strip-backup/b18e25de2cf5-1fd0a4ba-rebase.hg
+  $ hg update null -q
 
   $ hg log -G -T '{rev} {desc} {bookmarks}'
   o  4 H BOOK-C BOOK-D BOOK-E