cp: fix -R recursion detection

The correct logic is a lot simpler than the previous iteration.  We
record the base fts_name to avoid having to worry about whether we
needed the root symlink name or not (as applicable), then we can simply
shift all of that logic to after path translation to make it less
fragile.

If we're copying to DNE, then we'll have swapped out the NULL root_stat
pointer and then attempted to recurse on it.  The previously nonexistent
directory shouldn't exist at all in the new structure, so just back out
from that tree entirely and move on.

The tests have been amended to indicate our expectations better with
subdirectory recursion.  If we copy A to A/B, then we expect to copy
everything from A/B/* into A/B/A/B, with exception to the A that we
create in A/B.

Reviewed by:	bapt
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D34655

bin/cp/tests/cp_test.sh

@@ -95,16 +95,25 @@ matching_srctgt_contained_body()
 	# Let's do the same thing, except we'll try to recursively copy foo into
 	# one of its subdirectories.
 	mkdir foo
+	ln -s foo coo
 	echo "qux" > foo/bar
-	mkdir foo/loo
 	mkdir foo/moo
-	mkdir foo/roo
+	touch foo/moo/roo
 	cp foo/bar foo/zoo
 
 	atf_check cp -R foo foo/moo
+	atf_check cp -RH coo foo/moo
 	atf_check -o inline:"qux\n" cat foo/moo/foo/bar
+	atf_check -o inline:"qux\n" cat foo/moo/coo/bar
 	atf_check -o inline:"qux\n" cat foo/moo/foo/zoo
-	atf_check -e not-empty -s not-exit:0 stat foo/moo/foo/moo
+	atf_check -o inline:"qux\n" cat foo/moo/coo/zoo
+
+	# We should have copied the contents of foo/moo before foo, coo started
+	# getting copied in.
+	atf_check -o not-empty stat foo/moo/foo/moo/roo
+	atf_check -o not-empty stat foo/moo/coo/moo/roo
+	atf_check -e not-empty -s not-exit:0 stat foo/moo/foo/moo/foo
+	atf_check -e not-empty -s not-exit:0 stat foo/moo/coo/moo/coo
 }
 
 atf_test_case matching_srctgt_link