deps: upgrade embedded zlib to version 1.2.11

The VM on Travis apparently will still proceed, but it's good practice.

The trusty dependencies are now hosted on Bintray.

When the MSVC_CRTDBG option is set by the developer, we will link in the
dbghelper library to enable memory lead detection in MSVC projects. We
are doing so by adding it to the variable `CMAKE_C_STANDARD_LIBRARIES`,
so that it is linked for every library and executable built by CMake.
But this causes our builds to fail with a linker error:

```
    LINK: fatal error LNK1104: cannot open file 'advapi32.lib;Dbghelp.lib'
```

The issue here is that we are treating the variable as if it were an
array of libraries by setting it via the following command:

```
    SET(CMAKE_C_STANDARD_LIBRARIES "${CMAKE_C_STANDARD_LIBRARIES}"
        "Dbghelp.lib")
```

The generated build commands will then simply stringify the variable,
concatenating all the contained libraries with a ";". This causes the
observed linking failure.

To fix the issue, we should just treat the variabable as a simple
string. So instead of adding multiple members, we just add the
"Dbghelp.lib" library to the existing string, separated by a space
character.

The current version of zlib bundled with libgit2 is version 1.2.8. This
version has several CVEs assigned:

- CVE-2016-9843
- CVE-2016-9841
- CVE-2016-9842
- CVE-2016-9840

Upgrade the bundled version to the current release 1.2.11, which has
these vulnerabilities fixes.

adding GIT_FILTER_VERSION to GIT_FILTER_INIT as part of convention

travis: replace use of deprecated homebrew/dupes tap

Test improvements

Introduce a new test suite "odb::backend::simple", which utilizes the
fake backend to exercise the ODB abstraction layer. While such tests
already exist for the case where multiple backends are put together, no
direct testing for functionality with a single backend exist yet.

The fake backend currently implements all reading functions except for
the `exists_prefix` one. Implement it to enable further testing of the
ODB layer.

The `search_object` function takes the OID length as one of its
parameters, where its maximum length is `GIT_OID_HEXSZ`. The `exists`
function of the fake backend used `GIT_OID_RAWSZ` though, leading to
only the first half of the OID being used when finding the correct
object.

In order to be able to test the ODB prefix functions, we need to be able
to detect ambiguous prefixes in case multiple objects with the same
prefix exist in the fake ODB. Extend `search_object` to detect ambiguous
queries and have callers return its error code instead of always
returning `GIT_ENOTFOUND`.

Initialization of the `git_proxy_options` structure is never tested
anywhere. Include it in our usual initialization test in
"core::structinit::compare".

A newly added test uses the `git_repository_new` function without the
corresponding header file being included. While this works due to the
compiler deducing the correct function signature, we should obviously
just include the function's declaration file.

While our debug builds on MSVC platforms already tune the code optimizer
to aid debugging code, all the other platforms still use the default
optimization level. This makes it hard for developers on these platforms
to actually debug code while maintaining his sanity due to optimizations
like inlined code, elided variables etc.

To help this common use case, we can simply follow the MSVC example and
turn off code optimization with "-O0" for debug builds. While it would
be preferable to instead use "-Og" supported by more modern compilers,
we cannot guarantee that this level is available on all supported
platforms.

In our code base, we have some occasions where we use the "_DEBUG"
preprocessor macro to enable additional code which should not be part of
release builds. While we define this flag on MSVC platforms, it is
guarded by the conditional `WIN32 AND NOT CYGWIN` on other platforms
since 19be3f9e (Improve MSVC compiler, linker flags, 2013-02-13). While
this condition can be fulfilled by the MSVC platform, it is never
encountered due to being part of the `ELSE` part of `IF (MSVC)`.

The intention of the conditional was most likely to avoid the
preprocessor macro on Cygwin platforms, but to include it on everthing
else. As such, the correct condition here would be `IF (NOT CYGWIN)`
instead. But digging a bit further, the condition is only ever used in
two places:

1. To skip the test in "core::structinit", which should also work on
   Cygwin.
2. In "src/win32/git2.rc", where it is used to set additional file
   flags. As this file is included in MSVC builds only, it cannot cause
   any harm to set "_DEBUG" on Cygwin here.

As such, we can simply drop the conditional and always set "-D_DEBUG" on
all platforms.

In commit 9f75a9ce (Turning on runtime checks when building debug under
MSVC., 2012-03-30), we introduced a comment "Precompiled headers", which
actually refers to no related commands. Seeing that the comment never
had anything to refer to, we can simply remove it here.

The formulae provided by the homebrew/dupes tap are deprecated since at
least April 4, 2017, with formulae having been migrated to
homebrew/core.

Replace the deprecated reference to "homebrew/dupes/zlib" with only
"zlib".

Read prefix tests

Allow creation of a configuration object in an in-memory repository

travis: install openssl explicitly

Given a wholly in-memory repository, ensure that we can create an
anonymous remote and perform actions on it.

When in an in-memory repository - without a configuration file - do not
fail to create a configuration object.

Disambiguate error values: return `GIT_ENOTFOUND` when the item cannot
exist in the repository (perhaps because the repository is inmemory or
otherwise not backed by a filesystem), return `-1` when there is a hard
failure.

Previous to pulling out and extending the fake backend, it was quite
cumbersome to write tests for very specific scenarios regarding
backends. But as we have made it more generic, it has become much easier
to do so. As such, this commit adds multiple tests for scenarios with
multiple backends for the ODB.

The changes also include a test for a very targeted scenario. When one
backend found a matching object via `read_prefix`, but the last backend
returns `GIT_ENOTFOUND` and when object hash verification is turned off,
we fail to reset the error code to `GIT_OK`. This causes us to segfault
later on, when doing a double-free on the returned object.

Right now, the fake backend is quite restrained in the way how it
works: we pass it an OID which it is to return later as well as an error
code we want it to return. While this is sufficient for existing tests,
we can make the fake backend a little bit more generic in order to allow
us testing for additional scenarios.

To do so, we change the backend to not accept an error code and OID
which it is to return for queries, but instead a simple array of OIDs
with their respective blob contents. On each query, the fake backend
simply iterates through this array and returns the first matching
object.

In order to make the fake backend more useful, we want to enable it
holding multiple object references. To do so, we need to decouple it
from the single fake OID it currently holds, which we simply move up
into the calling tests.

The fake backend used by the test suite `odb::backend::nonrefreshing` is
useful to have some low-level tests for the ODB layer. As such, we move
the implementation into its own `backend_helpers` module.

smart_protocol: fix parsing of server ACK responses

odb_read_prefix: reset error in backends loop

When looking for an object by prefix, we query all the backends so that
we can ensure that there is no ambiguity.  We need to reset the `error`
value between backends; otherwise the first backend may find an object
by prefix, but subsequent backends may not.  If we do not reset the
`error` value then it will remain at `GIT_ENOTFOUND` and `read_prefix_1`
will fail, despite having actually found an object.

Update version number to 0.26