push: Prepare pack before sending pack header.

For large pushes, preparing the pack can take a while. Currently we
send the pack header first, followed by preparing the pack and then
finally sending the pack. Unfortunately github.com will terminate
a git-receive-pack command over http if it is idle for more than 10
seconds. This is easily exceeded for a large push, and so the push is
rejected with a Broken Pipe error.

This patch moves the pack preparation ahead of sending the pack header,
so that the timeout is avoided.

prepare_pack() can be called multiple times but will only do the work
once, so the original PREPARE_PACK call inside git_packbuilder_foreach()
remains.

Deprecate the `git_packfile_hash` function.  Callers should use the new
`git_packfile_name` function which provides a unique packfile name.

libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Separate the concerns of the hash functions from the git_oid functions.
The git_oid structure will need to understand either SHA1 or SHA256; the
hash functions should only deal with the appropriate one of these.

Makes usage of CLOCK_MONOTONIC conditional and makes functions that uses
git__timer handle clock resynchronization.

Call gettimeofday with tzp set to NULL as required by
https://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html