Options/Parser: add '--long-version' that adds debug information (#712)

Option for debugging and bugreporting purposes.

Current output:
```
Version: 0.9.1
Commit: 2dc211314e
  date: Sat Sep 12 13:31:59 2020 +0300
  branch: 2020-09-12-add-GitRev-to-version
```

Also date and branch is to direct contributors attention to updating (or having
in mind) that.

M  hnix.cabal
M  src/Nix/Options/Parser.hs

hnix.cabal

@@ -398,6 +398,7 @@ library
     , exceptions >= 0.10.0 && < 0.11
     , filepath >= 1.4.2 && < 1.5
     , free >= 5.1 && < 5.2
+    , gitrev >= 1.1.0 && < 1.4
     , hashable >= 1.2.5 && < 1.4
     , hashing >= 0.1.0 && < 0.2
     , hnix-store-core >= 0.1.0 && < 0.3

src/Nix/Options/Parser.hs

@@ -1,3 +1,5 @@
+{-# LANGUAGE TemplateHaskell #-}
+
 module Nix.Options.Parser where
 
 import           Control.Arrow                  ( second )
@@ -8,8 +10,11 @@ import qualified Data.Text                     as Text
 import           Data.Time
 import           Nix.Options
 import           Options.Applicative     hiding ( ParserResult(..) )
-import           Data.Version                   (showVersion)
-import           Paths_hnix                     (version)
+import           Data.Version                   ( showVersion )
+import           Development.GitRev             ( gitCommitDate
+                                                , gitBranch
+                                                , gitHash )
+import           Paths_hnix                     ( version )
 
 decodeVerbosity :: Int -> Verbosity
 decodeVerbosity 0 = ErrorsOnly
@@ -183,12 +188,29 @@ nixOptions current =
           )
         )
 
+--  2020-09-12: CLI --version option mechanism is tied to meta modules specificly generated by Cabal. It is possible to use Template Haskell to resolve the version, as also a g
 versionOpt :: Parser (a -> a)
-versionOpt = infoOption
-              (showVersion version)
-              (  long "version"
-              <> help "Show release version"
-              )
+versionOpt = shortVersionOpt <*> debugVersionOpt
+ where
+  shortVersionOpt :: Parser (a -> a)
+  shortVersionOpt = infoOption
+    (showVersion version)
+    (  long "version"
+    <> help "Show release version"
+    )
+
+  debugVersionOpt :: Parser (a -> a)
+  debugVersionOpt = infoOption
+    ( concat
+        [ "Version: ", showVersion version
+        , "\nCommit: ", $(gitHash)
+        , "\n  date: ", $(gitCommitDate)
+        , "\n  branch: ", $(gitBranch)
+        ]
+    )
+    (  long "long-version"
+    <> help "Show long debug version form"
+    )
 
 nixOptionsInfo :: UTCTime -> ParserInfo Options
 nixOptionsInfo current = info (helper <*> versionOpt <*> nixOptions current)