Code review comment for lp:~vila/bzr/284038-push-strict

This complements my first attempt to fix the bug while also fixing bugs #322808 and #65286,
by making --strict defaults to true.

Changing the default value revealed a bug in the previous implementation as push can be used without
a working tree.

Also the --strict option is meaningless if the --revision option is used.

Previous reviews mentioned introducing:
- a way to query config files for a bool option,
- a new method on working tree for checking changes.

Since both are also needed for bug #206577, I'd like to postpone these modifications until both fixes
have landed.