Improve BUILDDIR handling with symlinks

If BUILDDIR is a local symlink to another directory in the same parent
directory (out -> out.angler), then using out and .. as relative paths
to get back and forth work.

But if BUILDDIR is a symlink to another directory altogether (out ->
/mnt/sdd/out.master), then we shouldn't be relying on relative paths (so
that the source directory can still be moved).

Change-Id: I946c8116090410ab2b935eafba9b6e96f5f2f1dd

reverse_path_test.py

@@ -0,0 +1,45 @@
+from __future__ import print_function
+
+import os
+import shutil
+import tempfile
+import unittest
+
+from reverse_path import reverse_path
+
+class TestReversePath(unittest.TestCase):
+    def setUp(self):
+        self.tmpdir = tempfile.mkdtemp()
+        os.chdir(self.tmpdir)
+
+    def tearDown(self):
+        shutil.rmtree(self.tmpdir)
+
+    def test_absolute(self):
+        self.assertEqual(self.tmpdir, reverse_path('/out'))
+
+    def test_relative(self):
+        os.mkdir('a')
+        os.mkdir('b')
+
+        self.assertEqual('..', reverse_path('a'))
+
+        os.chdir('a')
+        self.assertEqual('a', reverse_path('..'))
+        self.assertEqual('.', reverse_path('../a'))
+        self.assertEqual('../a', reverse_path('../b'))
+
+    def test_symlink(self):
+        os.mkdir('b')
+        os.symlink('b', 'a')
+        os.mkdir('b/d')
+        os.symlink('b/d', 'c')
+
+        self.assertEqual('..', reverse_path('a'))
+        self.assertEqual('..', reverse_path('b'))
+        self.assertEqual(self.tmpdir, reverse_path('c'))
+        self.assertEqual('../..', reverse_path('b/d'))
+
+
+if __name__ == '__main__':
+    unittest.main()