Mixed revision working copy check when merging

Hi,

In Subversion 1.8 automatic merge doesn't allow merging to mixed
revision working copy by default. This is good.

The only problem that checks for mixed revision working copy performed
at the last stage. Considering the fact that users got mixed revision
after every commit it's very annoying to wait while svn merge contacts
server, asks for merge info and etc just to show error that you have
mixed revision working copy. I changed this behavior in r1488183 to
check working copy as soon as possible.

Change is very simple. I think it should be merged before next RC
otherwise Subversion 1.8.0 could get very bad perception from users.

-- 
Ivan Zhakov
CTO | VisualSVN | http://www.visualsvn.com