Validate repository directory ownership (v1.4)

Introduce the `GIT_OPT_SET_OWNER_VALIDATION` option, so that users can
disable repository ownership validation.

Obey the `safe.directory` configuration variable if it is set in the
global or system configuration. (Do not try to load this from the
repository configuration - to avoid malicious repositories that then
mark themselves as safe.)

Pull the global configuration loader out of the symlink check so that it
can be re-used.

Test that we prevent opening directories that are not owned by
ourselves.

Provide a mock for file ownership for testability.

Ensure that the repository directory is owned by the current user; this
prevents us from opening configuration files that may have been created
by an attacker.

Provide individual file ownership checks for both the current user and
the system user, as well as a combined current user and system user
check.

v1.4.2

Allow for no Git for Windows installation.  When there is no GfW found
in the path or registry, `git_win32__find_system_dirs` would return a
`GIT_ENOTFOUND`.  Callers were not expecting this.  Since this is no
error, we simply return `0` so that callers can move on with their
lives.

We use `git_error_set_after_callback_function` to determine whether
`update_tips` returned an error but do not store its return value making us
think it always returns 0.

Fix it by adding the common patter of storing it inside the `if` when calling it.

v1.4.1

We've added a lovely abstraction layer in xdiff so that it can call our
allocation functions.  But it also needs to call our free functions.  We
missed some `free` calls in `xmerge.c`.  Update them to use `xdl_free`.
Without this, we will pass a pointer allocated with a custom allocator
to the system free function.  💣

Fix building against system http-parser library by fixing
the find_package() argument.  It seems to have been accidentally changed
from HTTPParser to HTTP_Parser in de178d36, effectively making
the build against system library fail to find it:

```
CMake Warning at cmake/SelectHTTPParser.cmake:3 (find_package):
  By not providing "FindHTTP_Parser.cmake" in CMAKE_MODULE_PATH this project
  has asked CMake to find a package configuration file provided by
  "HTTP_Parser", but CMake did not find one.

  Could not find a package configuration file provided by "HTTP_Parser" with
  any of the following names:

    HTTP_ParserConfig.cmake
    http_parser-config.cmake

  Add the installation prefix of "HTTP_Parser" to CMAKE_PREFIX_PATH or set
  "HTTP_Parser_DIR" to a directory containing one of the above files.  If
  "HTTP_Parser" provides a separate development package or SDK, be sure it
  has been installed.
Call Stack (most recent call first):
  src/CMakeLists.txt:97 (include)

CMake Error at cmake/SelectHTTPParser.cmake:11 (message):
  http-parser support was requested but not found
Call Stack (most recent call first):
  src/CMakeLists.txt:97 (include)
```

libgit2 v1.4

Fix crashes in example programs on Windows (sprintf_s not compatible with snprintf)

merge: fix overlap between GIT_MERGE_FILE_FAVOR__CONFLICTED and GIT_MERGE_FILE_SIMPLIFY_ALNUM

meta: add a release configuration file

diff_file: fix crash if size of diffed file changes in workdir

Let's let GitHub handle our release notes as much as possible:
https://docs.github.com/en/repositories/releasing-projects-on-github/automatically-generated-release-notes

mktmp: improve our temp file creation

We made the flags that enable recursive merge internal, on the
assumption that nobody would want them and they're hard to reason about.
(Giving people an option that nobody wants is just extra noise.)

However, it made it hard for _us_ to reason about.  There's no good
reason to keep it private, let's just make it public and push that
cognitive load onto our poor users.  But they should expect it, they're
dealing with git, after all.

When we know that we know a file's size, and the file's size changes,
fail.

When we know the file size (because we're producing it from a working
directory iterator, or an index with an up-to-date cache) then set a
flag indicating as such.  This removes the ambiguity about a 0 file
size, which could indicate that a file exists and is 0 bytes, or that we
haven't read it yet.

Move the empty tree ID into a global space so that it can be generally
used.

This test was also provided by @jorio
https://github.com/libgit2/libgit2/pull/6208#issuecomment-1034072050