Merge branch 'jk/git-hash-cleanups'

Merge branch 'jk/git-hash-cleanups'

The 'git_hash_*()' wrappers have been updated to be used consistently
across the codebase instead of direct calls to members of 'struct
git_hash_algo', and 'git_hash_discard()' has been made idempotent to
simplify cleanups.

* jk/git-hash-cleanups:
  hash: check ctx->active flag in all wrapper functions
  http: use idempotent git_hash_discard()
  csum-file: use idempotent git_hash_discard()
  hash: make git_hash_discard() idempotent
  hash: document function pointers and wrappers
  hash: convert remaining direct function calls
  hash: use git_hash_init() consistently