pack.h: improve docs on how to create a packfile
tests/pack: do strict check of testpack's SHA1 hash
Merge pull request #1362 from schu/packbuilder-nits

packbuilder: minor improvements
tests/pack: fixup 6774b10

Initialize the hash ctx with git_hash_ctx_init, not git_hash_init.
hash: remove git_hash_init from internal api

Along with that, fix indentation in tests-clar/object/raw/hash.c
Merge pull request #1363 from schu/hash-initialization

hash: remove git_hash_init from internal api
Revert "hash: remove git_hash_init from internal api"

This reverts commit efe7fad6c96a3d6197a218aeaa561ec676794499, except for
the indentation fixes.