AU: When checking if we've visited a file before, don't follow symlinks.

A bug caused a failure when a file existed on the old image that was a
symlink to a non-existent path. This CL fixes that bug.

BUG=chromium-os:12393
TEST=unittests/generated and tested a delta

Change-Id: Ifba711be2bfc3c1d96a4ed8536b16b5d7b8a0cae

Review URL: http://codereview.chromium.org/6551015
diff --git a/delta_performer_unittest.cc b/delta_performer_unittest.cc
index bf4dcc7..586d582 100755
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -260,6 +260,9 @@
                                      "mv %s/tmp %s/srchardlink1",
                                      b_mnt.c_str(), b_mnt.c_str(),
                                      b_mnt.c_str(), b_mnt.c_str()).c_str()));
+    EXPECT_EQ(0, system(StringPrintf("rm %s/boguslink && "
+                                     "echo foobar > %s/boguslink",
+                                     b_mnt.c_str(), b_mnt.c_str()).c_str()));
     EXPECT_TRUE(utils::WriteFile(StringPrintf("%s/hardtocompress",
                                               b_mnt.c_str()).c_str(),
                                  reinterpret_cast<const char*>(kRandomString),