Merge branch 'ds/omit-trailing-hash-in-index'

Introduce an optional configuration to allow the trailing hash that
protects the index file from bit flipping.

* ds/omit-trailing-hash-in-index:
  features: feature.manyFiles implies fast index writes
  test-lib-functions: add helper for trailing hash
  read-cache: add index.skipHash config option
  hashfile: allow skipping the hash function

t/test-lib-functions.sh

@@ -1816,3 +1816,11 @@ test_cmp_config_output () {
 	sort config-actual >sorted-actual &&
 	test_cmp sorted-expect sorted-actual
 }
+
+# Given a filename, extract its trailing hash as a hex string
+test_trailing_hash () {
+	local file="$1" &&
+	tail -c $(test_oid rawsz) "$file" |
+		test-tool hexdump |
+		sed "s/ //g"
+}