support git files as input to computations

Using GIT keys, like are used when exporting git files to special
remotes. Except here the GIT key refers to a file checked into the git
repo.

Note that, since the compute remote uses catObject to get the content,
a symlink that is checked into git does not get followed. This is important
for security, because following a symlink and adding the content to the
repo as an annex object would allow exfiltrating content from outside
the repository.

Instead, the behavior with a symlink is to run the computation on the
symlink target. This may turn out to be confusing, and it might be worth
addcomputed checking if the file in git is a symlink and erroring out.
Or it could follow symlinks as long as the destination is a file in the
repisitory.

Git/Types.hs

@@ -145,7 +145,7 @@ newtype RefDate = RefDate String
 
 {- Types of objects that can be stored in git. -}
 data ObjectType = BlobObject | CommitObject | TreeObject
-	deriving (Show)
+	deriving (Show, Eq)
 
 readObjectType :: S.ByteString -> Maybe ObjectType
 readObjectType "blob" = Just BlobObject