BaseTools/Scripts: Ignore Mergify merge commits in PatchCheck.py

BaseTools/Scripts: Ignore Mergify merge commits in PatchCheck.py

Mergify adds merge commits to a PR when processing PRs using
the queue feature with auto rebase.  Update PatchCheck.py
to ignore commit message issues with these merge commits.
These merge commits are not added to the base branch when
the PR is merged by Mergify.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>