Remaining parts of https://github.com/JuliaLang/julia/blob/8d47a314537779c8fb86642c54925613628a91b0/deps/patches/libgit2-mbedtls-fixup.patch

Taken from https://github.com/JuliaLang/julia/blob/8d47a314537779c8fb86642c54925613628a91b0/deps/patches/libgit2-mbedtls-verify.patch, with some modifications.

Otherwise REQUIRED means that `git_stream_certificate` will always error.
We're doing the mbedtls check in verify_server_cert though.

remote/proxy: fix git_transport_certificate_check_db description

Flag options in describe.h as being optional

diff: Add missing GIT_DELTA_TYPECHANGE -> 'T' mapping.

appveyor: fix typo in registry key to disable DHE

This adds the 'T' status character to git_diff_status_char() for diff
entries that change type.

Commit 723e1e97 (appveyor: disable DHE to avoid spurious failures,
2018-03-29) added a workaround to fix spurious test failures due to a
bug in Windows' SChannel implementation. The workaround only worked by
accident, though, as the registry key was in fact mistyped. Fix the
typo.

Fix build with LibreSSL 2.7

appveyor: workaround for intermittent test failures

sha1dc: update to fix errors with endianess

LibreSSL 2.7 adds OpenSSL 1.1 API

Signed-off-by: Bernard Spil <brnrd@FreeBSD.org>

submodule: check index for path and prefix before adding submodule

odb: mempack: fix leaking objects when freeing mempacks

Our CI builds have intermittent failures in our online tests, e.g. with
the message "A provided buffer was too small". This is not a programming
error in libgit2 but rather an error in the SChannel component of
Windows. Under certain circumstances involving Diffie-Hellman key
exchange, SChannel is unable to correctly handle input from the server.
This bug has already been fixed in recent patches for Windows 10 and
Windows Server 2016, but they are not yet available for AppVeyor.

Manually pamper over that issue by disabling all ciphersuites using DHE
via the registry. While this disables more ciphers than necessary, we
really don't care for that at all but just want to avoid build failures
due to that bug.

See [1], [2] or [3] for additional information.

1: https://github.com/aws/aws-sdk-cpp/issues/671
2: https://github.com/dotnet/corefx/issues/7812
3: https://support.microsoft.com/en-us/help/2992611/ms14-066-vulnerability-in-schannel-could-allow-remote-code-execution-n

There are multiple references to undefined functions in the Microsoft
builds. Add headers to make them known.

When a ODB mempack gets free'd, we take no measures at all to free its
contents, most notably the objects added to the database, resulting in a
memory leak. Call `git_mempack_reset` previous to freeing the ODB
structures themselves, which takes care of releasing all associated
data structures.

This updates the version of SHA1DC to c3e1304ea3.

types: remove unused git_merge_result

`git_merge_result` is currently unused in the codebase and generates a blank page in the [documentation](https://libgit2.github.com/libgit2/#HEAD/type/git_merge_result).