Merge branch 'jk/alternate-ref-optim'

Merge branch 'jk/alternate-ref-optim'

Optimizes resource usage while enumerating refs from alternate
object store, to help receiving end of "push" that hosts a
repository with many "forks".

* jk/alternate-ref-optim:
  receive-pack: avoid duplicates between our refs and alternates
  receive-pack: treat namespace .have lines like alternates
  receive-pack: fix misleading namespace/.have comment
  receive-pack: use oidset to de-duplicate .have lines
  add oidset API
  fetch-pack: cache results of for_each_alternate_ref
  for_each_alternate_ref: replace transport code with for-each-ref
  for_each_alternate_ref: pass name/oid instead of ref struct
  for_each_alternate_ref: use strbuf for path allocation
  for_each_alternate_ref: stop trimming trailing slashes
  for_each_alternate_ref: handle failure from real_pathdup()