transports: ssh: disconnect session before freeing it 

revwalk: fix uninteresting revs sometimes not limiting graphwalk

attr_file: fix handling of directory patterns with trailing spaces

When we want to limit our graphwalk, we use the heuristic of checking
whether the newest limiting (uninteresting) revision is newer than the
oldest interesting revision. We do so by inspecting whether the first
item's commit time of the user-supplied list of revisions is newer than
the last added interesting revision. This is wrong though, as the user
supplied list is in no way guaranteed to be sorted by increasing commit
dates. This could lead us to abort the revwalk early before applying all
relevant limiting revisions, outputting revisions which should in fact
have been hidden.

Fix the heuristic by instead checking whether _any_ of the limiting
commits was made earlier than the last interesting commit. Add a test.

transports: local: fix assert when fetching into repo with symrefs

When comparing whether a path matches a directory rule, we pass the
both the path and directory name to `fnmatch` with
`GIT_ATTR_FNMATCH_DIRECTORY` being set. `fnmatch` expects the pattern to
contain no trailing directory '/', which is why we try to always strip
patterns of trailing slashes. We do not handle that case correctly
though when the pattern itself has trailing spaces, causing the match to
fail.

Fix the issue by stripping trailing spaces and tabs for a rule previous
to checking whether the pattern is a directory pattern with a trailing
'/'. This replaces the whitespace-stripping in our ignore file parsing
code, which was stripping whitespaces too late. Add a test to catch
future breakage.

When fetching into a repository which has symbolic references via the
"local" transport we run into an assert. The assert is being triggered
while we negotiate the packfile between the two repositories. When
hiding known revisions from the packbuilder revwalk, we unconditionally
hide all references of the local refdb. In case one of these references
is a symbolic reference, though, this means we're trying to hide a
`NULL` OID, which triggers the assert.

Fix the issue by only hiding OID references from the revwalk. Add a test
to catch this issue in the future.

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 function `libssh2_session_startup` has been deprecated since libssh2
version 1.2.8 in favor of `libssh2_session_handshake` introduced in the
same version. libssh2 1.2.8 was released in April 2011, so it is already
seven years old. It is available in Debian Wheezy, Ubuntu Trusty and
CentOS 7.4, so the most important and conservative distros already have
it available. As such, it seems safe to just use the new function.

The function `ssh_stream_free` takes over the responsibility of closing
channels and streams just before freeing their memory, but it does not
do so for the session. In fact, we never disconnect the session
ourselves at all, as libssh2 will not do so itself upon freeing the
structure. Quoting the documentation of `libssh2_session_free`:

    > Frees all resources associated with a session instance. Typically
    > called after libssh2_session_disconnect_ex,

The missing disconnect probably stems from a misunderstanding what it
actually does. As we are already closing the TCP socket ourselves, the
assumption was that no additional disconnect is required. But calling
`libssh2_session_disconnect` will notify the server that we are cleanly
closing the connection, such that the server can free his own resources.

Add a call to `libssh2_session_disconnect` to fix that issue.

[1]: https://www.libssh2.org/libssh2_session_free.html

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