Need to bump repos format number for 1.4.0

We need to bump the repos format number for 1.4.

This is because of the svndiff1 stuff. The svndiff1 change is signalled
by an increase of the *fs* format number from 1 to 2.

However, fs format numbers were not introduced until Subversion 1.2,
thus the possibility that 1.0 or 1.1 could naively act on a svndiff1
repository.

I brought the issue up with DannyB on IRC, and he was pretty confident
that operations would fail (somewhat) cleanly because of the specifics
of the svndiff0 to svndiff1 change, but I feel queasy on relying on
particulars of the change to guard against problems.

Therefore, I think we need to:
* have 1.4 libsvn_repos create new repositories at format 4, except if
--no-svndiff1 is specified, in which case format 3 is used.

* have 1.4 libsvn_repos accept both format 3 and format 4 repositories.

All ok? Clearly this needs to go into 1.4.0.

Max.