Merge branch 'jk/hash-algo-leak-fixes'

Merge branch 'jk/hash-algo-leak-fixes'

Various code paths that initialize a cryptographic hash context but
bail out or finish without calling 'git_hash_final()' have been taught
to call 'git_hash_discard()' to release allocated resources, fixing
memory leaks when Git is built with non-default backends like
'OpenSSL' or 'libgcrypt'.

* jk/hash-algo-leak-fixes:
  hash: add platform-specific discard functions
  hash: fix memory leak copying sha256 gcrypt handles
  http: discard hash in dumb-http http_object_request
  check_stream_oid(): discard hash on read error
  patch-id: discard hash when done
  csum-file: provide a function to release checkpoints
  csum-file: always finalize or discard hash
  hash: add discard primitive
  csum-file: drop discard_hashfile()