libgit2 can now isolate its home directory, and our test runner (by
default) isolates the home directory. In our CI environment, we want to
set up some pieces (like ssh configuration) in a fake homedir. Continue
to do so and propagate that to clar.

Use a dummy home directory for tests to avoid user data leaking into
test execution.

Now that we've split the notion of the home directory from the global
configuration store, our tests should use the appropriate one based on
what they're doing.

Some callers -- like our test suite and the test suites of our language
bindings -- want to isolate the home directory to avoid accidentally
including the executing user's actual home directory data.

Previously, we combined the notion of a home directory and global
configuration -- now that this is separated, we provide users the
ability to configure both.

Users can specify a config file to be included in the home directory
using `~/filename` syntax. Instead of looking in the global
configuration location (which may be overridden), use the user's
_actual_ home directory. This allows callers to change the global
configuration location separately from the home directory.

Users can specify an attribute file to be included in the home
directory using `~/filename` syntax. Instead of looking in the
global configuration location (which may be overridden), use the
user's _actual_ home directory. This allows callers to change the
global configuration location separately from the home directory.

I spent an hour banging my head against this, when it was because the
remote didn't trust my key.

Use `git_sysdir_find_homedir_file` to identify the path to the home
directory's `.ssh/known_hosts`; this takes Windows paths into account by
preferring `HOME`, then falling back to `HOMEPATH` and `USERPROFILE`
directories.

Provide a mechanism to look up the user's home directory, using the same
mechanism that we use for locating the global configuration path (a
fancy name for saying "the home directory").

SSH known hosts lookups now use this, instead of simply looking at the
HOME environment variable, to support Windows-style home directory
lookups in `HOME`, `HOMEPATH`, or `USERPROFILE`.

The windows sysdir functions are libgit2-specific and for git
compatibility only; remove them from the shared util directory and bring
them into the libgit2 source tree.

Support OpenSSL3: add OpenSSL deprection warning compatibility flag

Add `OPENSSL_API_COMPAT=0x10100000L` since we use the now-deprecated
low-level hash functions.

ci: always create test summaries, even on failure

When the dependent jobs fail -- possibly due to test failures -- we
should still produce the job summary that shows those test failures.

tests: update clar test runner

Abstract time counter for tests; use gettimeofday on Unix and
GetTickCount on Windows.

Update to the latest main version of clar, which includes improved xml
summary output.

src: hide unused hmac() prototype

The builtin hash uses the code verbatim from rfc6234, including
prototypes for functions that we don't use (like hmac). Remove all
unused prototypes to avoid collisions with things that an operating
system might provide (like hmac).

It conflicts with NetBSD's in its libc.

Closes #6457

push: use resolved oid as the source

211c9719 attempts to use the parsed OID
but inverted the arguments to `git_oid_cpy`.

push: revparse refspec source, so you can push things that are not refs

thread: avoid warnings when building without threads

transport: fix capabilities calculation

ci: update version numbers of actions

`git_clone` checks for existence of (non-empty) directories that would clash with what is about to be cloned.

This is problematic when cloning submodules since they make sense in the context of a parent module, so they should not use the current working dir.

Since in `git_submodule_update` we clone the submodule only when it is not yet initialized we do not need to perform directory checks.

Fixes #6433: git_submodule_update fails to update configured but missing submodule

This looks like a typo to me, from what i can see these lines were
added at the same time and because of how capabilities are calculated,
it's likely that this code will work in situations where these
capabilities were the last ones.

If a submodule has been configured but not yet added, do not try to update it.

Issue #6433: git_submodule_update fails to update configured but missing submodule

Verify that trying to update submodule which has been configured but not added does return an error.

Issue #6433: git_submodule_update fails to update configured but missing submodule

`git__noop` takes no arguments, so a simple `#define func(a) git__noop`
will produce warnings about the unused `a`. Introduce `git__noop_args`
to swallow arguments and avoid that warning.