dirstate: track that changes are pending in a transaction
Nothing is currently broken because if this, but this make the
`_invalidated_context` attribute more accurate.
Being more accurate here will help us later, when dealing with `status` call.
#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)