Merge branch 'mh/check-ref-format-3'

Merge branch 'mh/check-ref-format-3'

* mh/check-ref-format-3: (23 commits)
  add_ref(): verify that the refname is formatted correctly
  resolve_ref(): expand documentation
  resolve_ref(): also treat a too-long SHA1 as invalid
  resolve_ref(): emit warnings for improperly-formatted references
  resolve_ref(): verify that the input refname has the right format
  remote: avoid passing NULL to read_ref()
  remote: use xstrdup() instead of strdup()
  resolve_ref(): do not follow incorrectly-formatted symbolic refs
  resolve_ref(): extract a function get_packed_ref()
  resolve_ref(): turn buffer into a proper string as soon as possible
  resolve_ref(): only follow a symlink that contains a valid, normalized refname
  resolve_ref(): use prefixcmp()
  resolve_ref(): explicitly fail if a symlink is not readable
  Change check_refname_format() to reject unnormalized refnames
  Inline function refname_format_print()
  Make collapse_slashes() allocate memory for its result
  Do not allow ".lock" at the end of any refname component
  Refactor check_refname_format()
  Change check_ref_format() to take a flags argument
  Change bad_ref_char() to return a boolean value
  ...