commit: allow to close branch when committing change over a closed head Otherwise, an explicit other commit become necessary, which seems both silly and verbose. This is useful when merging closed heads on the same branches, for example when merging multiple repositories together.

#require emacs
  $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
  >  -f ert-run-tests-batch-and-exit
  Running 1 tests (*) (glob)
     passed  1/1  hg-test-mode--compilation-mode-support* (glob)
  
  Ran 1 tests, 1 results as expected* (glob)