tools/wltests: fix test_series's branch names resolution
Created by: derkling
When branch names are used instead of SHA1 in a series file, the name is resolved to a full SHA1 string which is longer then the 12 chars actually returned by the for-each-ref command we use to lookup names.
Ensure to always compare up to 12 chars of a specified SHA1.
Signed-off-by: Patrick Bellasi patrick.bellasi@arm.com