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.

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.

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

Backport fixes for CVE 2018-11235

We might modify caches due to us trying to load the configuration to figure out
what kinds of filesystem protections we should have.

We still compare case-insensitively to protect more thoroughly as we don't know
what specifics we'll see on the system and it's the behaviour from git.

When dealing with `core.proectNTFS` and `core.protectHFS` we do check
against `.gitmodules` but we still have a failing test as the non-filesystem
codepath does not check for it.

Any part of the library which asks the question can pass in the mode to have it
checked against `.gitmodules` being a symlink.

This is particularly relevant for adding entries to the index from the worktree
and for checking out files.

This is so we have it available for the path validity checking. In a later
commit we will start rejecting `.gitmodules` files as symlinks.

We may take in names from the middle of a string so we want the caller to let us
know how long the path component is that we should be checking.

We want to reject these as they cause compatibility issues and can lead to git
writing to files outside of the repository.

These will be used by the checkout code to detect them for the particular
filesystem they're on.

These can't go into the public API yet as we don't want to introduce API or ABI
changes in a security release.

Given a path component it knows what to pass to the filesystem-specific
functions so we're protected even from trees which try to use the 8.3 naming
rules to get around us matching on the filename exactly.

The logic and test strings come from the equivalent git change.

It checks against the 8.3 shortname variants, including the one which includes
the checksum as part of its name.

This lets us check for other kinds of reserved files.

Otherwise we would also admit `..\..\foo\bar` as a valid path and fail to
protect Windows users.

Ideally we would check for both separators without the need for the copied
string, but this'll get us over the RCE.

If the we decide that the "name" of the submodule (i.e. its path inside
`.git/modules/`) is trying to escape that directory or otherwise trick us, we
ignore the configuration for that submodule.

This leaves us with a half-configured submodule when looking it up by path, but
it's the same result as if the configuration really were missing.

The name check is potentially more strict than it needs to be, but it lets us
re-use the check we're doing for the checkout. The function that encapsulates
this logic is ready to be exported but we don't want to do that in a security
release so it remains internal for now.

We should pretend such submdules do not exist as it can lead to RCE.

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.

online tests: update auth for bitbucket test

Update the settings to use a specific read-only token for accessing our
test repositories in Bitbucket.

Refactor `gitno_extract_url_parts`

RFC 3986 says that hostnames can be percent encoded.  Percent decode
hostnames in our URLs.

Now that we can decode percent-encoded strings as part of `git_buf`s,
use that decoder in `gitno_extract_url_parts`.

Use `git_buf_decode_percent` so that we can avoid allocating a temporary
buffer.

Introduce a function to take a percent-encoded string (URI encoded,
described by RFC 1738) and decode it into a `git_buf`.