Begin handling for the MERGE method.
Begin handling for the MERGE method.

(open_txn): new function. factored out of dav_svn_checkout. given a name, it
  opens the svn_fs_txn_t object.

(dav_svn_checkout): use new open_txn function.

(dav_svn_merge): new function to perform a DAV MERGE

(struct dav_svn_hooks_vsn): add new merge hook

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion