Users can now select which regex implementation they want to use: one of
the system `regcomp_l`, the system PCRE, the builtin PCRE or the
system's `regcomp`.

By default the system `regcomp_l` will be used if it exists, otherwise
the system PCRE will be used.  If neither of those exist, then the
builtin PCRE implementation will be used.

The system's `regcomp` is not used by default due to problems with
locales.

Move some win32 type definitions to a standalone file so that they can
be included before other header files try to use the definitions.

PCRE includes compatibility functions that may go unused.  Don't warn.

The '[[:digit:]]' and '[[:alpha:]]' classes require double brackets, not
single.

The regex functions return nonzero (not necessarily negative values) on
failure.

In order to avoid us being unable to match characters which are part of
the normal US alphabet in certain weird languages, add two tests to
catch this behavior.

In order to make it easier adding more locale-related tests, add a
generalized framework handling initial setup of languages as well as the
cleanup of them afterwards.

While we already have a test for `p_regexec` with `LC_CTYPE` being
modified, `regexec` also alters behavior as soon as `LC_COLLATE` is
being modified. Most importantly, `LC_COLLATE` changes the way how
ranges are interpreted to just not handling them at all. Thus, ensure
that either we use `regcomp_l` to avoid this, or that we've fallen back
to our builtin regex functionality which also behaves properly.

While the test asserts that the error value indcates a non-value, it is
actually never getting assigned to. Fix this.

When searching for a configuration key for the diff driver, we construct
the config key by modifying a buffer and then passing it to
`git_config_get_multivar_foreach`. We do not check though whether the
modification of the buffer actually succeded, so we could in theory end
up passing the OOM buffer to the config function.

Fix that by checking return codes. While at it, switch to use
`git_buf_PUTS` to avoid repetition of the appended string to calculate
its length.

Use PCRE 8.42 as the builtin regex implementation, using its POSIX
compatibility layer.  PCRE uses ASCII by default and the users locale
will not influence its behavior, so its `regcomp` implementation is
similar to `regcomp_l` with a C locale.

Use the system includes (defined by libgit2) as the fuzzer includes.
The fuzzers link against internal libgit2 API and therefore need to have
the full include path that libgit2 uses.

Prefix all the calls to the the regexec family of functions with `p_`.
This allows us to swap out all the regular expression functions with our
own implementation.  Move the declarations to `posix_regex.h` for
simpler inclusion.

Support symlinks for directories in win32

rebase: orig_head and onto accessors

This is not implemented and should fail, but it should also not leak. To
allow the memory debugger to find leaks and fix this one we test this.

If someone passes just one ref (i.e. "master") and misses passing the
range we should be nice and return an error code instead of crashing.

cmake: correctly detect if system provides `regcomp`

Correctly write to missing locked global config

When linking against bundled libraries, we include their header
directories by using "-isystem". The reason for that is that we
want to handle our vendored library headers specially, most
importantly to ignore warnings generated by including them. By
using "-isystem", though, we screw up the order of searched
include directories by moving those bundled dependencies towards
the end of the lookup order. Like this, chances are high that any
other specified include directory contains a file that collides
with the actual desired include file.

Fix this by not treating the bundled dependencies' include
directories as system includes. This will move them to the front
of the lookup order and thus cause them to override
system-provided headers. While this may cause the compiler to
generate additional warnings when processing bundled headers,
this is a tradeoff we should make regardless to fix builds on
systems hitting this issue.

We assume that if we are on Win32, Amiga OS, Solaris or SunOS,
that the regcomp(3P) function cannot be provided by the system.
Thus we will in these cases always include our own, bundled regex
sources to make a regcomp implementation available. This test is
obviously very fragile, and we have seen it fail on MSYS2/MinGW
systems, which do in fact provide the regcomp symbol. The effect
is that during compilation, we will use the "regex.h" header
provided by MinGW, but use symbols provided by ourselves. This
in fact may cause subtle memory layout issues, as the structure
made available via MinGW doesn't match what our bundled code
expects.

There's one more problem with our regex detection: on the listed
platforms, we will incorrectly include the bundled regex code
even in case where the system provides regcomp_l(3), but it will
never be used for anything.

Fix the issue by improving our regcomp detection code. Instead of
relying on a fragile listing of platforms, we can just use
`CHECK_FUNCTION_EXISTS` instead. This will not in fact avoid the
header-ordering problem. But we can assume that as soon as a
system-provided "regex.h" header is provided, that
`CHECK_FUNCTION_EXISTS` will now correctly find the desired
symbol and thus not include our bundled regex code.

Opening a default config when ~/.gitconfig doesn't exist, locking it,
and attempting to write to it causes an assertion failure.

Treat non-existent global config file content as an empty string.

[RFC] util: introduce GIT_DOWNCAST macro

examples: implement SSH authentication

The rebase struct stores fields with information about the current
rebase process, which were not accessible via a public interface.

Accessors for getting the `orig_head` and `onto` branch
names and object ids have been added.

git_repository_init: stop traversing at windows root

Stop traversing the filesystem at the Windows directory root.  We were
calculating the filesystem root for the given directory to create, and
walking up the filesystem hierarchy.  We intended to stop when the
traversal path length is equal to the root path length (ie, stopping at
the root, since no path may be shorter than the root path).

However, on Windows, the root path may be specified in two different
ways, as either `Z:` or `Z:\`, where `Z:` is the current drive letter.
`git_path_dirname_r` returns the path _without_ a trailing slash, even
for the Windows root.  As a result, during traversal, we need to test
that the traversal path is _less than or equal to_ the root path length
to determine if we've hit the root to ensure that we stop when our
traversal path is `Z:` and our calculated root path was `Z:\`.

config_file: check result of git_array_alloc

git_array_alloc can return NULL if no memory is available, causing
a segmentation fault in memset. This adds GIT_ERROR_CHECK_ALLOC
similar to how other parts of the code base deal with the return
value of git_array_alloc.

In some parts of our code, we make rather heavy use of casting
structures to their respective specialized implementation. One
example is the configuration code with the general
`git_config_backend` and the specialized `diskfile_header`
structures. At some occasions, it can get confusing though with
regards to the correct inheritance structure, which led to the
recent bug fixed in 2424e64c (config: harden our use of the
backend objects a bit, 2018-02-28).

Object-oriented programming in C is hard, but we can at least try
to have some checks when it comes to casting around stuff. Thus,
this commit introduces a `GIT_CONTAINER_OF` macro, which accepts
as parameters the pointer that is to be casted, the pointer it
should be cast to as well as the member inside of the target
structure that is the containing structure. This macro then tries
hard to detect mis-casts:

- It checks whether the source and target pointers are of the
  same type. This requires support by the compiler, as it makes
  use of the builtin `__builtin_types_compatible_p`.

- It checks whether the embedded member of the target structure
  is the first member. In order to make this a compile-time
  constant, the compiler-provided `__builtin_offsetof` is being
  used for this.

- It ties these two checks together by the compiler-builtin
  `__builtin_choose_expr`. Based on whether the previous two
  checks evaluate to `true`, the compiler will either compile in
  the correct cast, or it will output `(void)0`. The second case
  results in a compiler error, resulting in a compile-time check
  for wrong casts.

The only downside to this is that it relies heavily on
compiler-specific extensions. As both GCC and Clang support these
features, only define this macro like explained above in case
`__GNUC__` is set (Clang also defines `__GNUC__`). If the
compiler is not Clang or GCC, just go with a simple cast without
any additional checks.