The `-Wdocumentation-deprecated-sync` option will warn when there is a
doxygen `\deprecated` tag but there is no corresponding deprecation
attribute on the function.  We want to encourage users to not use
particular APIs by marking them deprecated in the documentation without
necessarily raising a compiler warning by marking an item as deprecated.

For testing, we may wish to use a man-in-the-middle proxy that can
inspect the CONNECT traffic to our test endpoints.  For this, we will
need to accept the proxy's certificate, which will not be valid for the
true endpoint.

Add a new environment variable, GITTEST_REMOTE_SSL_NOVERIFY to disable
https certificate validation for the tests.

We previously used cURL to support HTTP proxies.  Now that we've added
this support natively, we can remove the curl dependency.

The implementations of git_openssl_stream_new and
git_mbedtls_stream_new have callers protected by #ifdefs and
are never called unless compiled in.  There's no need for a
dummy implementation.  Remove them.

Natively support HTTPS connections through proxies by speaking CONNECT
to the proxy and then adding a TLS connection on top of the socket.

Introduce `git_tls_stream_wrap` which will take an existing `stream`
with an already connected socket and begin speaking TLS on top of it.
This is useful if you've built a connection to a proxy server and you
wish to begin CONNECT over it to tunnel a TLS connection.

Also update the pluggable TLS stream layer so that it can accept a
registration structure that provides an `init` and `wrap` function,
instead of a single initialization function.

Rename credential callback to proxy_cred_cb to match new cert callback.

Give the proxy tests a proxy certificate callback, and allow self-signed
certificates when the `GITTEST_REMOTE_PROXY_SELFSIGNED` environment
variable is set (to anything).  In that case, simply compare the hostname
from the callback to the hostname that we connected to.

Store the error message from the underlying TLS library before calling
the certificate callback.  If it refuses to act (demonstrated by
returning GIT_PASSTHROUGH) then restore the error message.  Otherwise,
if the callback does not set an error message, set a sensible default
that implicates the callback itself.

Refactor certificate checking so that it can easily be called for
proxies or the remote server.

Create a simple data structure that contains information about the
server being connected to, whether that's the actual remote endpoint
(git server) or an intermediate proxy.  This allows for organization of
streams, authentication state, etc.

Put a limit on the number of authentication replays in the HTTP
transport.  Standardize on 7 replays for authentication or redirects,
which matches the behavior of the WinHTTP transport.

Teach the HTTP transport how to prompt for proxy credentials.

Prepare credential handling to understand both git server and proxy
server authentication.

Factor credential handling into its own function.  Additionally, add
safety checks to ensure that we are in a valid state - that we have
received a valid challenge from the server and that we have
configuration to respond to that challenge.

The HTTP transport should understand how to apply proxies when
configured with `GIT_PROXY_SPECIFIED` and `GIT_PROXY_SPECIFIED`.
When a proxy is configured, the HTTP transport will now connect
to the proxy (instead of directly to the git server), and will
request the properly-formed URL of the git server endpoint.

Rename `http_subtransport->io` to `http_subtransport->gitserver_stream`
to clarify its use, especially as we might have additional streams (eg
for a proxy) in the future.

As we want to support HTTPS proxies, support an optional
`GITTEST_REMOTE_PROXY_SCHEME` environment variable for tests that will
allow for HTTPS support.  (When unset, the tests default to HTTP
proxies.)

Change the `GITTEST_REMOTE_PROXY_URL` environment variable to be
`GITTEST_REMOTE_PROXY_HOST`, since it is a host:port combination, not an
actual URL.  (We cannot use a URL here since we may want to include the
username:password combination in the constructed URL.)

Rename the `connection_data` struct member to `gitserver_data`, to
disambiguate future `connection_data`s that apply to the proxy, not the
final server endpoint.

Allow certificate and credential callbacks to decline to act

Fix warning C4133 incompatible types in MSVC

Introduced in commit b433a22a.

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

Allow credential and certificate checking callbacks to return
GIT_PASSTHROUGH, indicating that they do not want to act.
Introduce this to support in both the http and ssh callbacks.
Additionally, enable the same mechanism for certificate validation.

This is most useful to disambiguate any meaning in the publicly exposed
credential and certificate functions (`git_transport_smart_credentials`
and `git_transport_smart_certificate_check`) but it may be more
generally useful for callers to be able to defer back to libgit2.

index: introduce git_index_iterator

commit: fix out-of-bound reads when parsing truncated author fields

While commit objects usually should have only one author field, our commit
parser actually handles the case where a commit has multiple author fields
because some tools that exist in the wild actually write them. Detection of
those additional author fields is done by using a simple `git__prefixcmp`,
checking whether the current line starts with the string "author ". In case
where we are handed a non-NUL-terminated string that ends directly after the
space, though, we may have an out-of-bounds read of one byte when trying to
compare the expected final NUL byte.

Fix the issue by using `git__prefixncmp` instead of `git_prefixcmp`.
Unfortunately, a test cannot be easily written to catch this case. While we
could test the last error message and verify that it didn't in fact fail parsing
a signature (because that would indicate that it has in fact tried to parse the
additional "author " field, which it shouldn't be able to detect in the first
place), this doesn't work as the next line needs to be the "committer" field,
which would error out with the same error message even if we hadn't done an
out-of-bounds read.

As objects read from the object database are always NUL terminated, this issue
cannot be triggered in normal code and thus it's not security critical.

Since we were not expecting this config entry to contain a string, we
would fail as soon as its (cached) value would be accessed. Hence,
provide some constants for the 4 states we use, and account for "always"
when we decide to reflog changes.

tests: 🌀 address two null argument instances

Some OpenSSL issues

worktree: Expose git_worktree_add_init_options

transport/http: Include non-default ports in Host header

Support symlinks on Windows when core.symlinks=true

Custom transports may want to ask libgit2 to invoke a configured
credential or certificate callback; however they likely do not know if a
callback was actually configured.  Return a sentinal value
(GIT_PASSTHROUGH) if there is no callback configured instead of crashing.

Provide a public git_index_iterator API that is backed by an index
snapshot.  This allows consumers to provide a stable iteration even
while manipulating the index during iteration.

strntol: fix out-of-bounds reads when parsing numbers with leading sign