With the metadata API: honor the doc by returning a NULL value if the  key doesn't exist.
With the metadata API: honor the doc by returning a NULL value if the 
key doesn't exist.

* subversion/libsvn_fs_base/tree.c
  (txn_body_metadata_get): Catch and handle the no-such-metadata error.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@873374 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion