Merge branch 'mh/ref-transaction'

Update "update-ref --stdin [-z]" and then introduce a transactional
support for (multi-)reference updates.

* mh/ref-transaction: (27 commits)
  ref_transaction_commit(): work with transaction->updates in place
  struct ref_update: add a type field
  struct ref_update: add a lock field
  ref_transaction_commit(): simplify code using temporary variables
  struct ref_update: store refname as a FLEX_ARRAY
  struct ref_update: rename field "ref_name" to "refname"
  refs: remove API function update_refs()
  update-ref --stdin: reimplement using reference transactions
  refs: add a concept of a reference transaction
  update-ref --stdin: harmonize error messages
  update-ref --stdin: improve the error message for unexpected EOF
  t1400: test one mistake at a time
  update-ref --stdin -z: deprecate interpreting the empty string as zeros
  update-ref.c: extract a new function, parse_next_sha1()
  t1400: test that stdin -z update treats empty <newvalue> as zeros
  update-ref --stdin: simplify error messages for missing oldvalues
  update-ref --stdin: make error messages more consistent
  update-ref --stdin: improve error messages for invalid values
  update-ref.c: extract a new function, parse_refname()
  parse_cmd_verify(): copy old_sha1 instead of evaluating <oldvalue> twice
  ...

contrib/examples/builtin-fetch--tool.c

@@ -31,7 +31,8 @@ static int update_ref_env(const char *action,
 		rla = "(reflog update)";
 	if (snprintf(msg, sizeof(msg), "%s: %s", rla, action) >= sizeof(msg))
 		warning("reflog message too long: %.*s...", 50, msg);
-	return update_ref(msg, refname, sha1, oldval, 0, QUIET_ON_ERR);
+	return update_ref(msg, refname, sha1, oldval, 0,
+			  UPDATE_REFS_QUIET_ON_ERR);
 }
 
 static int update_local_ref(const char *name,