Merge branch 'jk/fast-export-anonym-alt'

"git fast-export --anonymize" learned to take customized mapping to
allow its users to tweak its output more usable for debugging.

* jk/fast-export-anonym-alt:
  fast-export: use local array to store anonymized oid
  fast-export: anonymize "master" refname
  fast-export: allow seeding the anonymized mapping
  fast-export: add a "data" callback parameter to anonymize_str()
  fast-export: move global "idents" anonymize hashmap into function
  fast-export: use a flex array to store anonymized entries
  fast-export: stop storing lengths in anonymized hashmaps
  fast-export: tighten anonymize_mem() interface to handle only strings
  fast-export: store anonymized oids as hex strings
  fast-export: use xmemdupz() for anonymizing oids
  t9351: derive anonymized tree checks from original repo