build: repopick: Support squashed commits for Change-Id detection

 * Commits are identified by their last Change-Id,
    but the parser only considers the first one found

 * Reverse the list to find the last Change-Id
    and avoid multiple repopick attempts on the same commit

RM-290

Change-Id: Ie0204245a47799f8ead21148e0b4e9356cf271eb