Users opt into using the cache

Why?
====

Calculating the SHA of the entire tree can be expensive; this shifts
reading from/writing to the cache to be configured via a switch in the
CLI.

In the future, it might make sense to store metadata about the repo,
including historical time to calculate both the SHA and non-cached
versions, to compare and choose which one to do intelligently.

app/Main.hs

@@ -17,6 +17,7 @@ data Options = Options
     , oAllLikelihoods :: Bool
     , oIgnoredPaths :: [String]
     , oGrouping :: CurrentGrouping
+    , oWithCache :: Bool
     }
 
 main :: IO ()
@@ -38,7 +39,7 @@ run options = withoutCursor $ do
     terms <- pure . lines =<< getContents
     renderHeader terms
 
-    results <- cached $ unlines <$> executeSearch (oSearchRunner options) terms
+    results <- withCache options $ unlines <$> executeSearch (oSearchRunner options) terms
     let response = parseLines results
 
     resetScreen
@@ -48,6 +49,10 @@ run options = withoutCursor $ do
 
     return ()
 
+withCache :: Options -> IO String -> IO String
+withCache Options{ oWithCache = True } = cached
+withCache Options{ oWithCache = False } = id
+
 withInfo :: Parser a -> String -> String -> String -> ParserInfo a
 withInfo opts h d f =
     info (helper <*> opts) $ header h <> progDesc d <> footer f
@@ -75,6 +80,7 @@ parseOptions =
     <*> parseAllLikelihoods
     <*> parseIgnorePaths
     <*> parseGroupings
+    <*> parseWithCache
 
 parseSearchRunner :: Parser SearchRunner
 parseSearchRunner =
@@ -134,3 +140,9 @@ parseGroupingOption = strOption $
     short 'g'
     <> long "group-by"
     <> help "[Allowed: directory, term, file, none] Group results"
+
+parseWithCache :: Parser Bool
+parseWithCache = switch $
+    short 'c'
+    <> long "with-cache"
+    <> help "Write to cache and read when available"