utils: checkstyle.py: Check trailers for Amendment commits

The commit trailers are checked as part of processing the commit message
with the newly introduced TrailersChecker.

This relies on the trailers property being correctly exposed by the
Commit object, and is implemented for the base Commit but not processed
for Amendment commits.

Refactor the trailer property handling to a helper function in the base
Commit class and make use of it with a newly added call to obtain the
existing Trailers from the most recent commit when using Amendment.

Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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