refactor git-annex branch log filename code into central location

Having one module that knows about all the filenames used on the branch
allows working back from an arbitrary filename to enough information about
it to implement dropping dead remotes and doing other log file compacting
as part of a forget transition.

Utility/Misc.hs

@@ -91,6 +91,12 @@ massReplace vs = go [] vs
 			go (replacement:acc) vs (drop (length val) s)
 		| otherwise = go acc rest s
 
+{- First item in the list that is not Nothing. -}
+firstJust :: Eq a => [Maybe a] -> Maybe a
+firstJust ms = case dropWhile (== Nothing) ms of
+	[] -> Nothing
+	(md:_) -> md
+
 {- Given two orderings, returns the second if the first is EQ and returns
  - the first otherwise.
  -