pack: continue zlib while we can make progress

Continue the zlib stream as long as we can make progress; stop when we
stop getting output _or_ when zlib stops taking input from us.

Cache the parsed submodule config when diffing

Make git__strntol64() ~70%* faster

winhttp: support optional client cert

This change uses compiler intrinsics to detect overflows instead of
using divisions to detect potential overflow. This makes the code faster
and makes it easier to read as a bonus side-effect!

Some of the things this quickens:

* Config parsing.
* Tree parsing.
* Smart protocol negotiation.

\* Measured by running `libgit2_clar` with `-fno-optimize-sibling-calls
-fno-omit-frame-pointer` under `perf(1)`:

```shell
$ perf diff --symbols=git__strntol64 --compute=ratio \
  --percentage=absolute baseline.data perf.data
\# Event 'cycles'
\#
\# Baseline           Ratio  Shared Object
\# ........  ..............  .............
\#
     0.25%        0.321836  libgit2_clar
```

Also repurposed an unused function and deleted another one.

This change makes that anything that calls `git_diff__from_iterators`
(any of the `git_diff_xxx` functions) only need to parse the
`.gitmodules` file once. This can be avoided by calling
`git_repository_submodule_cache_all(...)`, but we can do that safely for
the user with no change in semantics.

Fixes: #5725

We require the library to be initialized with git_libgit2_init before it
is functional.  However, if a user tries to uses the library without
doing so - as they might when getting started with the library for the
first time - we will likely crash.

This commit introduces some guard rails - now instead of having _no_
allocator by default, we'll have an allocator that always fails, and
never tries to set an error message (since the thread-local state is
set up by git_libgit2_init).  We've modified the error retrieval
function to (try to) ensure that the library has been initialized before
getting the thread-local error message.

(Unfortunately, we cannot determine if the thread local storage has
actually been configured, this does require initialization by
git_libgit2_init.  But a naive attempt should be good enough for most
cases.)