don't report mergeinfo capability if repository doesn't support it

We added RA capabilities checks in 1.5.0-dev so that clients can avoid
operations that the RA layer doesn't support and such.  Those capabilities
checks are based strictly on Subversion library versions at the moment.  That
works fine for some of our features/capabilities, but not for mergeinfo.

Proper mergeinfo support does require 1.5-era Subversion libraries, but also
requires that the repository itself support mergeinfo storage and indexing. 
Since we don't auto-upgrade repositories, it is possible to run 1.5-era libs
against a 1.4-compatible (and therefore, mergeinfo-ignorant) repository.  That
causes the RA capability check for mergeinfo to claim support where none
actually exists.  And my momma taught me that lying is naughty.

So, we need to add capabilities queries to the svn_fs and svn_repos layers, too,
and then teach the RA layer to consult those before making claims about its own
capabilities.