64-bit Appveyor Builds

Clarified git_repository_is_empty() documentation

config: add parsing and getter for paths

Allow passing a NULL index to git_repository_set_index()

Fixed typo in git_repository_reinit_filesystem() documentation

More coverity fixes

Make sure sys/repository.h includes the required headers

Fixed git_repository_set_bare() not setting "core.bare" correctly

openssl: Add all required includes for AF_INET6 and in6_addr.

describe example: fix memory allocation size

pathspec: do not try to dereference NULL

We need to allocate memory for sizeof(char *) * ncommits, not just for
ncommits.

Issue detected by GCC's AddressSanitizer.

pathspec_match_free() should not dereference a NULL passed to it.

I found this issue when I tried to run example log program with
nonexistent branch:

./example/log help

Such call leads to segmentation fault.

This fixes the build at least on FreeBSD, where those types were not
defined indirectly:

src/openssl_stream.c:100:18: error: variable has incomplete type 'struct in6_addr'
        struct in6_addr addr6;
                        ^
src/openssl_stream.c:100:9: note: forward declaration of 'struct in6_addr'
        struct in6_addr addr6;
               ^
src/openssl_stream.c:111:18: error: use of undeclared identifier 'AF_INET'
        if (p_inet_pton(AF_INET, host, &addr4)) {
                        ^
src/unix/posix.h:31:40: note: expanded from macro 'p_inet_pton'
                                       ^
src/openssl_stream.c:115:18: error: use of undeclared identifier 'AF_INET6'
                if(p_inet_pton(AF_INET6, host, &addr6)) {
                               ^
src/unix/posix.h:31:40: note: expanded from macro 'p_inet_pton'
                                       ^

It was missing "common.h" and "types.h" like other system headers.
This generated compilation errors if including it directly.

This is supported by the underlying set_index() implementation
and setting the repository index to NULL is recommended by the
git_repository_set_bare() documentation.

checkout: free last created directory

The structinit tests don't make sense unless structure padding
is uniformly initialized, which is unlikely to happen on release
builds.  Only enable them for DEBUG builds.  Further, rename them
to core::structinit.

Add 64 bit and always build with default calling conventions, to
avoid trying to build with stdcall on amd64.

Remove logically dead code (we're already asserting)

README: Point to libgit2 account's AppVeyor badge

On case insensitive filesystems, we may have files in the working
directory that case fold to a name we want to write.  Remove those
files (by default) so that we will not end up with a filename that
has the unexpected case.

Symbolic links that abuse case insensitivity to write into .git.

Don't bother trying to recreate the previously created directory
during checkout, for a modest reduction in the number of stats.

Walk up the tree to mkdir, which is less immediately efficient,
but allows us to look at intermediate directories that may need
attention.

The documentation for `git_path_join_unrooted` states that the base
length will be returned, so that consumers like checkout know where
to start creating directories instead of always creating directories
at the directory root.

Checkout can now provide performance data about the number of (some)
syscalls performed using an optional callback.