Conflict markers should match EOL style in conflicting files

use consistent names for the #include / #define header guard pattern.

Upgrade xdiff to git's most recent version, which includes changes to
CR/LF handling.  Now CR/LF included in the input files will be detected
and conflict markers will be emitted with CR/LF when appropriate.

In commit a390a8464 (cmake: move defines into "features.h" header,
2017-07-01), we have introduced a new "features.h" header. This file is
being generated by the CMake build system based on how the libgit2 build
has been configured, replacing the preexisting method of simply setting
the defines inside of the CMake build system. This was done to help
splitting up the build instructions into multiple separate
subdirectories.

An overlooked shortcoming of this approach is that some projects making
use of libgit2 build the library with custom build systems, without
making use of CMake. For those users, the introduction of the
"features.h" file makes their life harder as they would have to also
generate this file.

Fix this issue by guarding all inclusions of the generated header file
by the `LIBGIT2_NO_FEATURES_H` define. Like this, other build systems
can skip the feature header and simply define all used features by
specifying `-D` flags for the compiler again.

In a future commit, we will split out the build instructions for our
library directory and move them into a subdirectory. One of the benefits
is fixing scoping issues, where e.g. defines do not leak to build
targets where they do not belong to. But unfortunately, this does also
pose the problem of how to propagate some defines which are required by
both the library and the test suite.

One way would be to create another variable keeping track of all added
defines and declare it inside of the parent scope. While this is the
most obvious and simplest way of going ahead, it is kind of unfortunate.
The main reason to not use this is that these defines become implicit
dependencies between the build targets. By simply observing a define
inside of the CMakeLists.txt file, one cannot reason whether this define
is only required by the current target or whether it is required by
different targets, as well.

Another approach would be to use an internal header file keeping track
of all defines shared between targets. While configuring the library, we
will set various variables and let CMake configure the file, adding or
removing defines based on what has been configured. Like this, one can
easily keep track of the current environment by simply inspecting the
header file. Furthermore, these dependencies are becoming clear inside
the CMakeLists.txt, as instead of simply adding a define, we now call
e.g. `SET(GIT_THREADSAFE 1)`.

Having this header file though requires us to make sure it is always
included before any "#ifdef"-preprocessor checks are executed. As we
have already refactored code to always include the "common.h" header
file before any statement inside of a file, this becomes easy: just make
sure "common.h" includes the new "features.h" header file first.

The current order of declarations and includes between "common.h" and
"w32_crtdbg_stacktrace.h" is rather complicated. Both header files make
use of things defined in the other one and are thus circularly dependent
on each other. This makes it currently impossible to compile the
"w32_crtdbg_stacktrace.c" file when including "common.h" inside of
"w32_crtdbg_stacktrace.h".

We can disentangle the mess by moving declaration of the inline crtdbg
functions into the "w32_crtdbg_stacktrace.h" file and adding additional
includes inside of it, such that all required functions are available to
it. This allows us to break the dependency cycle.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

The old pthread-file did re-implement the pthreads API with exact symbol
matching. As the thread-abstraction has now been split up between Unix- and
Windows-specific files within the `git_` namespace to avoid symbol-clashes
between libgit2 and pthreads, the rewritten wrappers have nothing to do with
pthreads anymore.

Rename the Windows-specific pthread-files to honor this change.

We commonly have to check if a git_buf has been allocated
correctly or if we ran out of memory. Introduce a new macro
similar to `GITERR_CHECK_ALLOC` which checks if we ran OOM and if
so returns an error. Provide a `#nodef` for Coverity to mark the
error case as an abort path.