Fix @Rev:<n> extra-deps specification not working (input-output-hk#90)

This mistake was introduced in e04dbd4.
Where previously the (+++) operator from
https://hackage.haskell.org/package/Cabal-2.4.1.0/docs/Distribution-Compat-ReadP.html#v:-43--43--43-
was used to give no precedence to left or right,
now the <|> operator is used instead, which fails if the left fails and
won't try the right (non-backtracking).

To fix this, a try is used,
which explicitly makes the code backtrack if the left was unsuccessful.

A better fix would be possible by reading until ':', then change
behavior depending on whether "sha256" or "rev" was read

Author: infinisil

lib/Stack2nix/Stack.hs

@@ -163,7 +163,7 @@ revSuffix :: ParsecParser CabalRev
 revSuffix = string "@rev:" *> (CabalRev . read <$> some (satisfy (`elem` ['0'..'9'])))
 
 suffix :: ParsecParser (Maybe (Either Sha256 CabalRev))
-suffix = option Nothing (Just <$> ((Left <$> sha256Suffix) <|> (Right <$> revSuffix)))
+suffix = option Nothing (Just <$> (try (Left <$> sha256Suffix) <|> (Right <$> revSuffix)))
 
 pkgIndex :: ParsecParser Dependency
 pkgIndex = PkgIndex <$> parsec <*> suffix <* eof