[benchmark] check digests for input files only (#1314)

Getting some weird crashes without this change:

```
Error when running Shake build system:
  at want, called at bench/hist/Main.hs:87:16 in main:Main
* Depends on: bench_example_HLS
  at need, called at src/Development/Benchmark/Rules.hs:154:37 in shake-bench-0.1.0.0-inplace:Development.Benchmark.Rules
* Depends on: bench-results/bench_example_HLS/HEAD/edit.heap.svg
* Raised the exception:
bench-results/bench_example_HLS/HEAD/edit.heap.svg: openFile: resource busy (file is locked)

Benchmark benchHist: ERROR
cabal: Benchmarks failed for bench:benchHist from ghcide-0.7.2.0.
```

Author: pepeiborra

Diff for: ghcide/bench/hist/Main.hs

@@ -66,7 +66,7 @@ type instance RuleResult GetExample = Maybe Example
 type instance RuleResult GetExamples = [Example]
 
 main :: IO ()
-main = shakeArgs shakeOptions {shakeChange = ChangeModtimeAndDigest, shakeThreads = 0} $ do
+main = shakeArgs shakeOptions {shakeChange = ChangeModtimeAndDigestInput, shakeThreads = 0} $ do
   createBuildSystem $ \resource -> do
       configStatic <- liftIO $ readConfigIO config
       let build = outputFolder configStatic