This change:

* Starts using GCC's and clang's `__atomic_*` intrinsics instead of the
  `__sync_*` ones, since the former supercede the latter (and can be
  safely replaced by their equivalent `__atomic_*` version with the
  sequentially consistent model).
* Makes `git_atomic64`'s value `volatile`. Otherwise, this will make
  ThreadSanitizer complain.
* Adds ways to load the values from atomics. As it turns out,
  unsynchronized read are okay only in some architectures, but if we
  want to be correct (and make ThreadSanitizer happy), those loads
  should also be performed with the atomic builtins.
* Fixes two ThreadSanitizer warnings, as a proof-of-concept that this
  works:
  - Avoid directly accessing `git_refcount`'s `owner` directly, and
    instead makes all callers go through the `GIT_REFCOUNT_*()` macros,
    which also use the atomic utilities.
  - Makes `pool_system_page_size()` race-free.

Part of: #5592

Fix error return for invalid extensions.

Change bare free to allocator free (fixes #5653)

The info pointer was allocated with git__malloc, so needs to be free'd with git__free.
This bug can lurk pretty easily since if there's no custom allocator this is fine.

midx: Introduce a parser for multi-pack-index files

This change is the first in a series to add support for git's
multi-pack-index. This should speed up large repositories significantly.

Part of: #5399

Fixed typo in comment

Fix binary diff showing /dev/null

Fix #5565

Pre-Android 5 did not implement a virtual filesystem atop FAT partitions for Unix permissions, which causes chmod to fail. However, Unix permissions have no effect on Android anyway as file permissions are not actually managed this way, so treating it as a no-op across all Android is safe.

httpclient: only free challenges for current_server type

Respect `init.defaultBranch` setting

patch_parse: handle absence of "index" header for new/deleted cases

boolean config parsing fails in some cases with mapped values

Fix config file parsing with multi line values containing quoted parts

Fix release build warnings

occured -> occurred

Signed-off-by: Sven Strickroth <email@cs-ware.de>

When trying to the template dir, we pass in a `NULL` pointer for the
filename. That's perfectly fine, but if we're failing to find the
template directory then we'll creat an error message with the `NULL`
pointer passed in.

Fix the issue by setting different error messages based on whether the
filename is given or not. This even makes sense, as we're not searching
for a file in case we have no `name`, but for a directory. So the error
would've been misleading anyway.

Fix deprecation links inside of documentation not working

When building libgit2 with the release build type, then GCC complains
about mode being potentially uninitialized. While this seems to be a
false positive, let's silence this warning by zero-initializing the
mode.

The `signature` variable in `git_reference_rename` isn't initialized and
neither does `git_reference__log_signature` always do. So if the latter
function fails, we'll call `git_signature_free` on this unininitialized
variable.

Fix the issue by initializing the pointer with `NULL`.

Fix typo: Make ifndef macroname the same as the define name

diff stats: fix segfaults with new files

WinHTTP: Try to use TLS1.3

Fixed includes for FreeBSD

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Don't fail if a HTTP server announces he supports a protocol upgrade

cf. RFC7230 section 6.7, an Upgrade header in a normal response merely informs the client that the server supports upgrading to other protocols, and the client can ask for such an upgrade in a later request. The server requiring an upgrade is via the 426 Upgrade Required response code, not the mere presence of the Upgrade response header.

(closes issue #5573)

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Return false instead of segfaulting when checking for default port

`default_port_for_scheme` returns NULL if the scheme is not one of the
builtin ones. This may cause a segmentation fault if a custom transport
URL happens to contain a port number, and this code path is triggered
(e.g. by setting git_fetch_options->update_fetchhead to 1).