Don't check symlinks for license headers

Because it tried to open the symlink and caused an error. So ignore symlinks.

BUG=None
TEST=tried uploading a CL with a symlink
TEST=`./pre-upload_unittest.py` passes

Change-Id: I07f2394a832d82e692fbe5ec4d3ec270a671dcaa
Reviewed-on: https://chromium-review.googlesource.com/188072
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index 59836ad..235b037 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -168,8 +168,8 @@
                         COMMON_EXCLUDED_PATHS)
 
   for f in files:
-    # Ignore non-existant files
-    if os.path.exists(f):
+    # Ignore non-existant files and symlinks
+    if os.path.exists(f) and not os.path.islink(f):
       contents = open(f).read()
       if not contents:
         # Ignore empty files