Fix string parsing of tab indentation

As much as everyone would wish for, tabs are not whitespace according to
Nix whitespace stripping.
Fixes NixOS#116, but my overall confidence level in Nixfmt's correctness in
parsing the mess which is the Nix syntax remains rather low.

src/Nixfmt/Parser.hs

@@ -292,6 +292,7 @@ stripIndentation parts = case commonIndentation $ mapMaybe lineHead parts of
   Nothing -> map (const []) parts
   Just indentation -> map (stripParts indentation) parts
 
+-- Merge adjacent string parts which resulted as parsing artifacts
 normalizeLine :: [StringPart] -> [StringPart]
 normalizeLine [] = []
 normalizeLine (TextPart "" : xs) = normalizeLine xs

src/Nixfmt/Util.hs

@@ -12,7 +12,7 @@ module Nixfmt.Util (
 )
 where
 
-import Data.Char (isAlpha, isDigit, isSpace)
+import Data.Char (isAlpha, isDigit)
 import Data.Text as Text (
   Text,
   all,
@@ -73,7 +73,7 @@ commonPrefix a b =
 -- prefix of zero texts is infinite, represented as Nothing.
 commonIndentation :: [Text] -> Maybe Text
 commonIndentation [] = Nothing
-commonIndentation [x] = Just $ Text.takeWhile isSpace x
+commonIndentation [x] = Just $ Text.takeWhile (== ' ') x
 commonIndentation (x : y : xs) = commonIndentation (commonPrefix x y : xs)
 
 isSpaces :: Text -> Bool