make fs_base's auto-merging more efficient

In svn 1.2, libsvn_fs_fs had it's auto-merging algorithm changed to avoid expensive predecessor walks.  
See revisions 13222, 13223, 13232.

This logic needs to be ported to libsvn_fs_base as well.   This issue should be marked with a 
"performance" keyword.  Specifically, it will make things more efficient when a new HEAD revision 
comes in during a commit that's the process of finalization.  It doesn't happen very often, but when it 
does, the current auto-merging algorithm is very expensive.