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).

submodule: check path and prefix before adding submodule


submodule: fix test errors

The git_describe_options in git_describe_commit and
git_describe_workdir and the git_describe_format_options in
git_describe_format are optional and can be NULL. State this in the
documentation to make people's lives easier when calling these
functions.

Signed-off-by: Remy Suen <remy.suen@gmail.com>

checkout: change default strategy to SAFE

As per #4200, our default is quite surprising to users that expect checkout to just "do the thing".

Add myself to git.git-authors

odb: fix writing to fake write streams

In commit 7ec7aa4a (odb: assert on logic errors when writing objects,
2018-02-01), the check for whether we are trying to overflowing the fake
stream buffer was changed from returning an error to raising an assert.
The conversion forgot though that the logic around `assert`s are
basically inverted. Previously, if the statement

    stream->written + len > steram->size

evaluated to true, we would return a `-1`. Now we are asserting that
this statement is true, and in case it is not we will raise an error. So
the conversion to the `assert` in fact changed the behaviour to the
complete opposite intention.

Fix the assert by inverting its condition again and add a regression
test.

Our mempack ODB backend has no test coverage at all right now. Add a
simple test suite to at least have some coverage of the most basic
operations on the ODB.