When a file doesn't exist in PREV, files should not have a source of "revision 0" in SVN < 1.8

Before SVN 1.8, if a file doesn't exist in the repository at the PREV revision and `-r PREV` is given, the PREV revision should be used -- not "revision 0".  In SVN 1.8.x, it should actually be "revision 0".  And in SVN 1.9.x, it should be "nonexistent".

Since you can't use `-r PREV` on a file that doesn't exist in PREV until SVN 1.7, this really only affects 1.7.x. 