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Description of problem:
diff -r blindly follows symlinks, namely directory symlinks.  When the
compared directories accidentaly contain link cycles, strange things
happen.  It's not possible to use diff -r at such directory structures at all.

How reproducible:
Always

Steps to Reproduce:
1. Create two identical directory structures with link cycles using
following commands:

  mkdir 1 2
  cd 1
  mkdir a b
  cd a
  ln -s ../b b
  cd ../b
  ln -s ../a a
  cd ../../2
  mkdir a b
  cd a
  ln -s ../b b
  cd ../b
  ln -s ../a a
  cd ../..

2. run

  diff -r 1 2
	

Actual Results:  For a good while it seems nothing is happening, then diff
prints two messages like (several hundreds of `a/b/'s omitted in the place
of `...')

   diff: 1/a/b/a/b/a/b/a/b/a/b/a/b/.../a/b/a/b/a/b: File name too long

and after a while it ends.

Expected Results:  diff realizes the directories are identical, realizes it
pretty fast and doesn't print any error messages.

Additional info:

It is arguable what diff should do in similar situations.  I don't see why
it should follow symlinks at all, namely directory symlinks.  Maybe it
could be made optional, at least.