gcc and clang support __builtin_add_overflow, use it whenever
possible, falling back to our naive routines.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

https://github.com/attractivechaos/klib/pull/42/files introduces
`kreallocarray`.  Hook that up as our `git__reallocarray`.

Add some helper functions to check for overflow in a type-specific
manner.

Ensure that the given length to `p_read` is of ssize_t and ensure
that callers test the return as if it were an `ssize_t`.

Use `size_t` to hold the size of arrays to ease overflow checking,
lest we check for overflow of a `size_t` then promptly truncate
by packing the length into a smaller type.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.

Introduce `git_buf_grow_by` to incrementally increase the size of a
`git_buf`, performing an overflow calculation on the growth.

Introduce git__reallocarray that checks the product of the number
of elements and element size for overflow before allocation.  Also
introduce git__mallocarray that behaves like calloc, but without the
`c`.  (It does not zero memory, for those truly worried about every
cycle.)

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

Checkout performance

Remove stale reflog namespace directory before branch creation

Ensure we can make a repo at the root of the filesystem

Add libssh2's library and include directories.

Large merge perf test.

Update some third-party copyrights

Fix doc comment formatting

Speed up AppVeyor builds.

AppVeyor build machines come with Python 2.7 and CMake 2.8 pre-installed and in the PATH.

repository: parse init.templatedir as a path

This is a path so we must use the path getter so we get the tilde
expansion done.

Update clar to d17c7f6

Don't try to strip trailing paths from the root directory on
Windows (trying to create `C:` will fail).

`p_stat` calls `git_win32_path_from_utf8`, which canonicalizes the
path.  Do not further try to modify the path, else we trim the
trailing slash from a root directory and try to access `C:` instead
of `C:/`.

Test to ensure that we can create a repository at the filesystem
root.  Introduces a new test environment variable,
`GITTEST_INVASIVE_FILESYSTEM` for tests that do terrible things like
escaping the clar sandbox and writing to the root directory.  It is
expected that the CI builds will enable this but that normal people
would not want this.

Handle 8dot3 filenames being disabled on Win32

for-each-ref example: initialize libgit2 in main

I don't get how it was working without git_libgit2_init() call. I run it
and libgit2 throws assertion somewhere in its internals. Now it works.

Updated commit with shutdown at the end.