Make wc_db.c's scan_addition() function follow the usual wc_db pattern that the
private apis are called within a transaction and don't obtain one itself.

This api is used by quite a lot of other functions that already have a lock and
don't need an additional savepoint.

* subversion/libsvn_wc/wc_db.c
  (scan_addition_txn): Rename to scan_addition and move comment here.
  (scan_addition): Remove old wrapper.
  (svn_wc__db_scan_addition,
   svn_wc__db_scan_moved): Use a lock around scan_addition.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545127 13f79535-47bb-0310-9956-ffa450edef68