Check for top == head at the start of every transaction

We used to check it in the run() method, but that's sometimes too
late: for example, it causes stg coalesce to ask for a commit message
_before_ the check, resulting in a lost commit message if the check
fails.

As before, the check can be disabled for the few commands that need
it.

Signed-off-by: Karl Hasselström <kha@treskal.com>