Markdown uses trailing whitespace to indicate context; eg, a blank line
with four spaces indicates that there is a hard break between the
(indented) lines surrounding that line.  Ensure that editors do not
remove this blank.

We've introduced a manifest for the clib version system that includes a
version number; we should update it at release time to correspond with
the version number in the header.

This PR introduces a new top-level file, `package.json`, which enables this repository compatibility with [`clib`](https://github.com/clibs/clib), an open source C package manager.  By doing this, users of `clib` can quickly include the `libgit2` library within their project.

path: unify `git_path_is_*` APIs

Fix negative gitignore rules with leading directories 

Custom memory allocators

Tie in the newly created infrastructure for swapping out memory
allocators into our settings code. A user can now simply use the new
option "GIT_OPT_SET_ALLOCATOR" with `git_libgit2_opts`, passing in an
already initialized allocator structure as vararg.

Currently, our memory allocators are being redirected to the correct
implementation at compile time by simply using macros. In order to make
them swappable at runtime, this commit reshuffles that by instead making
use of a global "git_allocator" structure, whose pointers are set up to
reference the allocator functions. Like this, it becomes easy to swap
out allocators by simply setting these function pointers.

In order to initialize a "git_allocator", our provided allocators
"stdalloc" and "crtdbg" both provide an init function. This is being
called to initialize a passed in allocator struct and set up its members
correctly.

No support is yet included to enable users of libgit2 to switch out the
memory allocator at a global level.