[PATCH] Merge source and target must be related but different branches, v1

Here's a patch to reject silly merge attempts, which up to now give silly results. This does not apply to all merges (general 2-URL and cherry-pick merges), but the commonly used 'sync' and 'reintegrate' forms of merge only make sense when the source and target 'branches' are related (have a common ancestor) and are not the same. This patch applies such checks to the 'reintegrate' merge, simple 'sync' merge, and the 'svn mergeinfo' command. A few tests currently fail with this patch -- tests that use the special "svn merge FILE[@REV]" syntax.  I'm currently investigating this and learning what it's supposed to be doing. Comments? - Julian