tools/check_copyright: modularize and improve readability

This patch simplifies the way the check_copyright.py script works
by creating helper functions and reducing the number of nested
for loops. It simplifies the process of retrieving file changes
and adds the possibility to set the commit hash from which to
compare.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I722ee086b6d615029fd5013de42cd2017c38660e