More backports to 0.21

FindFirstFile will fail with INVALID_HANDLE_VALUE if there are no
children to the given path, which can happen if the given path is a
file (and obviously has no children) or if the given path is an empty
mount point.  (Most directories have at least directory entries '.'
and '..', but ridiculously another volume mounted in another drive
letter's path space do not, and thus have nothing to enumerate.)

If FindFirstFile fails, check if this is a directory-like thing
(a mount point).

A reparse point that is an IO_REPARSE_TAG_MOUNT_POINT could be
a junction or an actual filesystem mount point.  (Who knew?)
If it's the latter, its reparse point will report the actual
volume information \??\Volume{GUID}\ and we should not attempt
to dereference that further, instead readlink should report
EINVAL since it's not a symlink / junction and its original
path was canonical.

Yes, really.

All versions of SSL are considered deprecated now, so let's ask OpenSSl
to only use TLSv1. We still ask it to load those ciphers for
compatibility with servers which want to use an older hello but will use
TLS for encryption.

For good measure we also disable compression, which can be exploitable,
if the OpenSSL version supports it.

Add a few backports to 0.21 maintenance

* Ensure alignment by copying the content into a structure on the stack

When we free the global state at thread termination, we must also free
the error message in order not to leak the string once per thread.

The getaddrinfo function indicates failure with a non-zero return code,
but this code is not necessarily negative. On platforms like Android
where the code is positive, a failed call causes libgit2 to segfault.

Since this isn't under sys/ we need to have it included in the main
git2.h file.

This fixes #2551.

A backported fix makes use of the new name for git_cherry_pick (namely
git_cherrypick), but we still need to use the old one in the maint
branch.

Passing 0 as the length of the paths to check to git_diff_index_to_workdir
results in all files being treated as conflicting, that is, all untracked or
modified files in the worktree is reported as conflicting

Added test case to illustrate bogus conflicts detected if no files were merged, and untracked files exist in the workdir.

When the call to the agent fails, we must retrieve the error message
just after the function call, as other calls may overwrite it.

As the agent authentication is the only one which has a teardown and
there does not seem to be a way to get the error message from a stored
error number, this tries to introduce some small changes to store the
error from the agent.

Clearing the error at the beginning of the loop lets us know whether the
agent has already set the libgit2 error message and we should skip it,
or if we should set it.

The callers of git_packfile_unpack() expect the obj_offset argument to
be set to the beginning of the next object. We were mistakenly returning
the the offset of the object's data, which causes the CRC function to
try to use the wrong offset.

Set obj_offset to curpos instead of elem->offset to point to the next
element and bring back expected behaviour.

The recv buffer (parse_buffer) and the buffer have independent sizes and
offsets. We try to fill in parse_buffer as much as possible before
passing it to the http parser. This is fine most of the time, but fails
us when the buffer is almost full.

In those situations, parse_buffer can have more data than we would be
able to put into the buffer (which may be getting full if we're towards
the end of a data sideband packet).

To work around this, we check if the space we have left on our buffer is
smaller than what could come from the network. If this happens, we make
parse_buffer think that it has as much space left as our buffer, so it
won't try to retrieve more data than we can deal with.

As the start of the data may no longer be at the start of the buffer, we
need to keep track of where it really starts (data_offset) and use that
in our calculations for the real size of the data we received from the
network.

This fixes #2518.