Define memory ownership and lifetime rules for what for-each-ref
feeds to its callbacks (in short, "you do not own it, so make a
copy if you want to keep it").

* mh/reflife: (25 commits)
  refs: document the lifetime of the args passed to each_ref_fn
  register_ref(): make a copy of the bad reference SHA-1
  exclude_existing(): set existing_refs.strdup_strings
  string_list_add_refs_by_glob(): add a comment about memory management
  string_list_add_one_ref(): rename first parameter to "refname"
  show_head_ref(): rename first parameter to "refname"
  show_head_ref(): do not shadow name of argument
  add_existing(): do not retain a reference to sha1
  do_fetch(): clean up existing_refs before exiting
  do_fetch(): reduce scope of peer_item
  object_array_entry: fix memory handling of the name field
  find_first_merges(): remove unnecessary code
  find_first_merges(): initialize merges variable using initializer
  fsck: don't put a void*-shaped peg in a char*-shaped hole
  object_array_remove_duplicates(): rewrite to reduce copying
  revision: use object_array_filter() in implementation of gc_boundary()
  object_array: add function object_array_filter()
  revision: split some overly-long lines
  cmd_diff(): make it obvious which cases are exclusive of each other
  cmd_diff(): rename local variable "list" -> "entry"
  ...