1.7 fix necessary? file externals in unversioned subdirs

Ran a test that puts a file external inside an unversioned subdir. As soon
as I have such a file external, running 'svn cleanup' (even though there's
nothing to clean up) fails:

[[[
svn: E155016: database inconsistency at local_relpath='X/UNV/xb' verifying
expression 'have_a_parent_row'
]]]

So we should fix, or enforce in 1.7.x that file externals can only be
created directly in versioned directories. Because if user has an
unversioned path between WC and file external, 'svn cleanup' always fails.

Reproduction:
[[[
echo a > a
svn add a
svn ci -mm

svn ps svn:externals "^/a UNVER/a" .
svn up

svn cleanup
]]]

It doesn't matter if UNVER exists prior to the update or not.

(also attached)

~Neels