Bug 264450

When we set a branch config value in git-webkit, that value is appended to any existing value for that branch. On PR branches, this isn't a problem because the user should only have a single bug associated with the branch. But for cherry-picks, multiple cherry-picks are very likely to be done on the same branch. Since the oldest values come first, the commit message template will tend to prefer the older value to the newer ones. Our commit message template should prefer newer values to older ones, and generally, should not allow duplicates of the same value. "not allowing duplicates" is complicated by the fact that a user may store a newline delinated value, which will look like two independent values. This is particularly relevant for bugs.