Merge branch 'mh/safe-create-leading-directories'

Code clean-up and protection against concurrent write access to the
ref namespace.

* mh/safe-create-leading-directories:
  rename_tmp_log(): on SCLD_VANISHED, retry
  rename_tmp_log(): limit the number of remote_empty_directories() attempts
  rename_tmp_log(): handle a possible mkdir/rmdir race
  rename_ref(): extract function rename_tmp_log()
  remove_dir_recurse(): handle disappearing files and directories
  remove_dir_recurse(): tighten condition for removing unreadable dir
  lock_ref_sha1_basic(): if locking fails with ENOENT, retry
  lock_ref_sha1_basic(): on SCLD_VANISHED, retry
  safe_create_leading_directories(): add new error value SCLD_VANISHED
  cmd_init_db(): when creating directories, handle errors conservatively
  safe_create_leading_directories(): introduce enum for return values
  safe_create_leading_directories(): always restore slash at end of loop
  safe_create_leading_directories(): split on first of multiple slashes
  safe_create_leading_directories(): rename local variable
  safe_create_leading_directories(): add explicit "slash" pointer
  safe_create_leading_directories(): reduce scope of local variable
  safe_create_leading_directories(): fix format of "if" chaining

builtin/init-db.c

@@ -515,13 +515,14 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 				saved = shared_repository;
 				shared_repository = 0;
 				switch (safe_create_leading_directories_const(argv[0])) {
-				case -3:
+				case SCLD_OK:
+				case SCLD_PERMS:
+					break;
+				case SCLD_EXISTS:
 					errno = EEXIST;
 					/* fallthru */
-				case -1:
-					die_errno(_("cannot mkdir %s"), argv[0]);
-					break;
 				default:
+					die_errno(_("cannot mkdir %s"), argv[0]);
 					break;
 				}
 				shared_repository = saved;