Don't fail when deleting a missing bzr/master reference

It is possible to interrupt a sync operation after the bzr/master
reference has been cancelled and before it has been re-created:
we can simply ignore errors when trying to delete it.
Don't sync if we're going to do an overwriting push

This make possible to do an push --overwrite after amending a
commit.
Log command invocations with loglevel=INFO

Leave loglevel=DEBUG for very verbose things like the fast-import
output data.
Merge pull request #44 from em-/master

Some fixes to be able to push --overwrite after amending a commit