Improve polling : rollback a baseline to track the revision has been rollback

This patch resolves the following problem.
If the build fails when at JOB of Jenkins, was the repository hg rollback in the workspace by JOB, polling will fail.

---
[workspace] $ hg status --rev cf99ef461cd787fcef9d32b4018f141634e2dfa0 --rev 4ba5c267c1ce68ef73c0ae4d2bae1ad8713d96b0
ERROR: Failed to run hg status --rev cf99ef461cd787fcef9d32b4018f141634e2dfa0 --rev 4ba5c267c1ce68ef73c0ae4d2bae1ad8713d96b0
abort: unknown revision 'cf99ef461cd787fcef9d32b4018f141634e2dfa0' !
FATAL: Failed to compare with remote repository