pack: cast the number of objects to size_t

CMake: Search for ssh2 instead of libssh2.

midx: allow unknown chunk ids in multi-pack index files

docs: fix IRC server from freenode to libera

This commit update the contributing documents with the
new IRC server and remove the old freenode.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Fixes "CMake Error: could not resolve ssh2" on Windows-MSVC.

Similar to a previous change where we had to change the casting when loading the
index file, we also need to make sure we don't restrict the numbers to 32-bit
when looking up objects in packfiles.

This was done about three years ago in git itself, but we never got he update in
this library.

These chunks work like extensions where it's fine not to know what one means. We
can skip over it and keep processing the file instead of erroring out.

When a worktree is not prunable, an error message will be set with
information about why. Document that.

PR #5712 predates several refactorings to move the utility code into a
more general purpose codebase. Update to reflect the refactorings.

Fixes #5598

Monsters.

`check_symbol_exists` is superior to `check_function_exists`; use it
consistently in our cmake configuration

Not all systems have poll(2); emulate it with select(2).

Make socket I/O non-blocking and add optional timeouts.

Users may now set `GIT_OPT_SET_SERVER_CONNECT_TIMEOUT` to set a shorter
connection timeout. (The connect timeout cannot be longer than the
operating system default.) Users may also now configure the socket read
and write timeouts with `GIT_OPT_SET_SERVER_TIMEOUT`.

By default, connects still timeout based on the operating system
defaults (typically 75 seconds) and socket read and writes block.

Add a test against our custom testing git server that ensures that we
can timeout reads against a slow server.

v0.6.0 of poxygit add support for throttling connections to test
timeouts and low-bandwidth situations.

The `gitno` buffer interface is another layer on top of socket reads.
Abstract it a bit into a "static string" that has `git_str` like
semantics but without heap allocation which moves the actual reading
logic into the socket / stream code, and allows for easier future usage
of a static / stack-allocated `git_str`-like interface.

We lose some error information from the read / write callbacks to
stransport. Store our own error value in the object so that we can
ensure that we rely upon it.

`git__timer` is now `git_time_monotonic`, and returns milliseconds
since an arbitrary epoch.

Using a floating point to store the number of seconds elapsed was
clever, as it better supports the wide range of precision from the
different monotonic clocks of different systems. But we're a version
control system, not a real-time clock.

Milliseconds is a good enough precision for our work _and_ it's the
units that system calls like `poll` take and that our users interact
with.

Make `git_time_monotonic` return the monotonically increasing number
of milliseconds "ticked" since some arbitrary epoch.

Thread-local storage: handle failure cases

Now that we've reduced the usage of GIT_THREADSTATE, remove it entirely
in favor of git_threadstate_get().

git_oid_tostr_s could fail if thread-local state initialization fails.
In that case, it will now return `NULL`.  Callers should check for
`NULL` and propagate the failure.

Thread-local storage data may fail to initialize; in this case, do not
try to set the error message into it.  When the thread state has not
been initialized, return a hardcoded message to that affect.

actions: simplify execution with composite action

Update xdiff to git 2.40.1's version

Work around -Werror problems when detecting qsort variants

Introduce `check_prototype_definition_safe` that is safe for `Werror`
usage.

`QSORT_R` and `QSORT_S` -> `QSORT`

Add `GIT_UNUSED_ARG` which is an attribute for arguments, for
compatibility with dependencies.

Shallow (#6396) with some fixes from review