bisect-kit: handle symbolic link case when removing session

BUG=b:183686737
TEST=manually

Change-Id: I891ec71f927479321ccfbc1492913df12154f91d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2874817
Tested-by: Zheng-Jie Chang <zjchang@chromium.org>
Auto-Submit: Zheng-Jie Chang <zjchang@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
diff --git a/setup_cros_bisect.py b/setup_cros_bisect.py
index e8c13c2..d736bbc 100755
--- a/setup_cros_bisect.py
+++ b/setup_cros_bisect.py
@@ -92,6 +92,8 @@
 
 
 def is_btrfs_subvolume(path):
+  if os.path.islink(path):
+    return False
   if util.check_output('stat', '-f', '--format=%T', path).strip() != 'btrfs':
     return False
   return util.check_output('stat', '--format=%i', path).strip() == '256'