quality(style): add task that installs a precommit hook that format code

`./gradlew installPrecommitHook` backs up existing hook
The hook formats all changed files (staged or unstaged) and outputs a
warning about other files that do not conform to google code style

quality/checkstyle/java.gradle

@@ -1,9 +1,13 @@
 apply plugin: 'checkstyle'
 
+import java.nio.file.Paths
+import java.nio.file.Files
+import java.nio.file.StandardCopyOption
+
 // see https://docs.gradle.org/current/dsl/org.gradle.api.plugins.quality.CheckstyleExtension.html
 checkstyle {
   toolVersion '8.10' // see https://github.com/checkstyle/checkstyle/releases
-  configDir rootProject.file('config/quality/checkstyle')
+  configDir rootProject.file(Paths.get('config', 'quality', 'checkstyle'))
   ignoreFailures false
   showViolations true
   maxErrors 0
@@ -33,3 +37,20 @@ task reformatJavaSources(
   outputs.files(sources)
 }
 
+task installPrecommitHook {
+  doLast {
+    def hookFile = file(Paths.get(rootDir.absolutePath, '.git', 'hooks', 'pre-commit'))
+    if(hookFile.exists()) {
+      Files.copy(
+          Paths.get(hookFile.absolutePath),
+          Paths.get(hookFile.parent, "${hookFile.name}.${new Date().format('yyyymmdd-HH:mm')}.backup")
+      )
+    }
+    Files.copy(
+        Paths.get(rootDir.absolutePath, 'config', 'quality', 'checkstyle', 'pre-commit'),
+        Paths.get(hookFile.absolutePath),
+        StandardCopyOption.REPLACE_EXISTING
+    )
+    hookFile.setExecutable(true)
+  }
+}

quality/checkstyle/pre-commit

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+# Git pre commit hook that reformats all staged and unstaged java files before commit.
+# Files that do not comply to the code format but were staged or unstagd warned but ignored..
+# The rationale is to enforce code style without creating "format all files" commits
+# accidentally because that cleanup should be separate in a separate commit.
+
+staged_files=$(git diff --name-only --cached)
+unstaged_files=$(git diff --name-only)
+
+# echo $unstaged_files | xargs git stash push -q
+./gradlew reformatJavaSources
+echo $staged_files | xargs git add # add all touched & reformatted files
+
+changed_files=$(git ls-files -m)
+# offenders = changed_files - unstaged_files
+union="$(echo "$changed_files" ; echo "$unstaged_files")"
+offenders="$(echo "$union" | sort | uniq -u)"
+if [[ $offenders ]]; then
+    echo "😬 The following files are not conforming to our style guide, commit them separately after running ./gradlew reformatJavaSources\n"
+    echo "$offenders"
+    echo ""
+fi
+
+echo $offenders | xargs git checkout # discard reformat of untouched files
+
+exit 0