Based on http://hackage.haskell.org/trac/ghc/ticket/3307 ,
whether FilePath contains decoded unicode varies by OS.
So, add a configure check for it.
Also, renamed showFile to filePathToString
Internally, the filenames are stored as un-decoded unicode.
I tried decoding them, but then haskell tries to access the wrong files.
Hmm.
So, I've unhappily chosen option "B", which is to decode filenames before
they are displayed.
Moved away from a map of flags to storing config directly in the AnnexState
structure. Got rid of most accessor functions in Annex.
This allowed supporting multiple --exclude flags.
