change spotless to apply only on staged changes, unless everything is staged

.githooks/pre-commit.sh

@@ -1,18 +1,25 @@
-#!/bin/sh
+#!/bin/bash
 
 everything_staged=$(git diff --name-only)
+# `no_partial_stages` from https://github.com/evilmartians/lefthook/issues/140#issuecomment-1012967850
+no_partial_stages=$(comm -12 <(git diff --name-only | sort) <(git diff --name-only --staged | sort))
+format_only_staged=-DspotlessFiles="$(git diff --staged --name-only | sed 's/^/.*/' | paste -sd ',' -)"
 
-if [ -n "$everything_staged" ]; then
-  # Don't commit because, after we reformat, the formatting will be unstaged, but it's too hard to determine (and maybe
-  # ambiguous) what are the formatting differences and what was originally unstaged.
-  mvn spotless:check || {
-    echo "*** ERROR ***"
-    echo "Detected a partial commit that doesn't have proper formatting."
-    echo "Run 'mvn spotless:apply', make sure every change you want to commit is added, and commit again."
+if [ -n "$no_partial_stages" ]; then
+  # Format only staged changes. Don't commit if we change anything, because we can't partially format a file, and it's
+  # ambiguous what formatting changes should be staged and what shouldn't.
+  mvn spotless:check "$format_only_staged" || {
+    mvn spotless:apply "$format_only_staged"
+    echo "Reformatted a partially-staged file. Re-interactively-stage and commit again."
     exit 1
   }
+  git add -u
+elif [ -n "$everything_staged" ]; then
+  # Format only staged changes. We must re-add them because the formats aren't committed.
+  mvn spotless:apply "$format_only_staged"
+  git diff --name-only --staged | tr \\n \\0 | xargs -0 git add -f
 else
-  # It's not ambiguous, just format and re-add everything tracked
+  # Format everything because spotless caches. We must re-add everything because the formats aren't committed.
   mvn spotless:apply
   git add -u
 fi