don't fail compilation if git is unavailable

(instead, `walint --version` will now just say "unknown")

src/Version.hs

@@ -6,10 +6,12 @@ module Version ( version ) where
 import           Control.Monad.Trans (liftIO)
 import qualified Language.Haskell.TH as TH
 import           System.Process      (readProcess)
+import GHC.IO (catchAny)
 
 version :: String
 version = "walint rc3 2021 (" <>
     $(do
-        hash <- liftIO $ readProcess "git" ["rev-parse", "HEAD"] ""
+        hash <- liftIO $ catchAny (readProcess "git" ["rev-parse", "HEAD"] "")
+                         (\_ -> pure "[unknown]")
         pure . TH.LitE . TH.StringL $ take 40 hash) ++
     ")"