deps: update pcre to 8.45

#6576 git_diff_index_to_workdir reverse now loads untracked content

tests: add allocator with limited number of bytes

Fixes #6344: git_branch_move now renames the reflog instead of deleting.

Partial fix for #6532: insert-by-date order.

midx: do not try to look at every object in the index

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 previous issues around doing way too much verification at loading
time, checking whether the object index chunk is monotonic is better left for
git-fsck instead of every single time we want to look at something in the
repository.

As midx files grow, this starts taking more and more times. As an example, I
went looking for this because it's taking about 1.5s to do a single object
lookup in a repository that's ended up with a 7G multi-pack-index file.

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

Avoid passing a `NULL` ptr to `memcpy` -- that's UB (even if size is 0)

Test that `git_buf` correctly fails if no more bytes can be allocated.
This is mostly for demonstration purposes.

In several circumstances, we get bug reports about things that happen in
situations where the environment is quite limited with regards to
available memory. While it's expected that functionality will fail if
memory allocations fail, the assumption is that we should do so in a
controlled way. Most importantly, we do not want to crash hard due to
e.g. accessing NULL pointers.

Naturally, it is quite hard to debug such situations. But since our
addition of pluggable allocators, we are able to implement allocators
that fail in deterministic ways, e.g. after a certain amount of bytes
has been allocated. This commit does exactly that.

To be able to properly keep track of the amount of bytes currently
allocated, allocated pointers contain tracking information. This
tracking information is currently limited to the number of bytes
allocated, so that we can correctly replenish them on calling `free` on
the pointer. In the future, it would be feasible to extend the tracked
information even further, e.g. by adding information about file and line
where the allocation has been performed. As this introduced some
overhead to allocations though, only information essential to limited
allocations is currently tracked.

Remove the number of functions that custom allocator users need to
provide; nobody should need to implement `substrdup`. Keep it to the
basics that are actually _needed_ for allocation (malloc, realloc,
free) and reimplement the rest ourselves.

In addition, move the failure check and error setting _out_ of the
custom allocators and into a wrapper so that users don't need to deal
with this. This also allows us to call our allocator (without the
wrapper) early so that it does not try to set an error on failure, which
may be important for bootstrapping.

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.