Most importantly, clarify what it means for HTTPS and SSH to be supported.

This partially reverts bdec62dc which activates
the transport code-paths which allow you to use a custom TLS implementation
without having to have one at build-time.

However the capabilities describe how libgit2 was built, not what it could
potentially support, bring back the ifdefs so we only say we support HTTPS if
libgit2 was itself built with a TLS implementation.

refdb: disable concurrent compress in the threading tests on Windows

Various fixes

This is far from an ideal situation, but this causes issues on Windows which
make it harder to develop anything, as these tests hit issues which relate
specifically to the Windows filesystem like permission errors for files we
should be able to access. There is an issue likely related to the ordering of
the repack, but there's enough noise that it does not currently help us to run
this aspect of the test in CI.

The function `cl_git_thread_check()` is defined as static. As the
function is defined in a header file which is included by our
tests, this can result in warnings for every test file where
`cl_git_thread_check` is never used.

Fix the issue by marking it as inline instead.

git_rebase_init: correctly handle detached HEAD

Properly pass `wchar *` type to giterr_set

remote: fix typo in git_fetch_init_options docs

git_rebase_finish relies on head_detached being set, but
rebase_init_merge was only setting it when branch->ref_name was unset.
But branch->ref_name would be set to "HEAD" in the case of detached
HEAD being either implicitly (NULL) or explicitly passed to
git_rebase_init.

Introduce some clar helpers for child threads

Don't `cl_git_pass` in a child thread.  When the assertion fails, clar
will `longjmp` to its error handler, but:

> The effect of a call to longjmp() where initialization of the jmp_buf
> structure was not performed in the calling thread is undefined.

Instead, set up an error context that threads can populate, and the
caller can check.

We want a predictable number of initializations in our multithreaded
init test, but we also want to make sure that we have _actually_
initialized `git_libgit2_init` before calling `git_thread_create` (since
it now has a sanity check that `git_libgit2_init` has been called).

Since `git_thread_create` is internal-only, keep this sanity check.
Flip the invocation so that we `git_libgit2_init` before our thread
tests and `git_libgit2_shutdown` again after.

Introduce `git_thread_exit`, which will allow threads to terminate at an
arbitrary time, returning a `void *`.  On Windows, this means that we
need to store the current `git_thread` in TLS, so that we can set its
`return` value when terminating.

We cannot simply use `ExitThread`, since Win32 returns `DWORD`s from
threads; we return `void *`.

use `giterr_set_str()` wherever possible

`giterr_set()` is used when it is required to format a string, and since
we don't really require it for this case, it is better to stick to
`giterr_set_str()`.

This also suppresses a warning(-Wformat-security) raised by the compiler.

Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>

Bump version number to v0.25

sortedcache: plug leaked file descriptor

curl_stream: use CURLINFO_ACTIVESOCKET if curl is recent enough

Introduce a GitHub Issue Template

CHANGELOG: fill in some updates we missed

The `CURLINFO_LASTSOCKET` information has been deprecated since
curl version 7.45.0 as it may result in an overflow in the
returned socket on certain systems, most importantly on 64 bit
Windows. Instead, a new call `CURLINFO_ACTIVESOCKET` has been
added which instead returns a `curl_socket_t`, which is always
sufficiently long to store a socket.

As we need to provide backwards compatibility with curl versions
smaller than 7.45.0, alias CURLINFO_ACTIVESOCKET to
CURLINFO_LASTSOCKET on platforms without CURLINFO_ACTIVESOCKET.

Plug a leak in the refs compressor