describe example: function to add commits to opts

Allow checkout to handle newly cloned repositories, remove `GIT_CHECKOUT_SAFE_CREATE`

We want to ignore GIT_ENOTFOUND, but for that we need to capture the
error code from the reflog deletion.

git_branch_delete() should ignore errors from non-existing reflogs

win32: add the patch level to the .dll fileversion

Win32 DLLs have four fields for the version number (major, minor,
teeny, patch).  If a consumer wants to build a custom DLL, it may
be useful to set the patchlevel version number in the DLL.

This value only affects the DLL version number, it does not affect
the resultant "version number", which remains major.minor.teeny.

Update to clar 2b73f5e

rebase: check alloc result

When the repository does not contain an index, emulate git's behavior
and upgrade to `SAFE_CREATE`.  This allows us to check out repositories
created with `git clone --no-checkout`.

README: provide some more explanation about TLS

Fix MAX 32 bit build problem described in libgit2/libgit2#2917

Fix race in git_packfile_unpack.

Streaming filters

Introduce GITTEST_INVASIVE_FS_STRUCTURE for things that are invasive
to your filesystem structure (like creating folders at your filesystem
root) and GITTEST_INVASIVE_FS_SIZE for things that write lots of data.

Refactor `git_filter_list__load_with_attr_reader` into
`git_filter_list__load_ext`, which takes a `git_filter_options`.

For consistency with the rest of the library, where an opt is an
options *structure*.

Provide a convenience function that creates a buffer that can be provided
to callers but will not be freed via `git_buf_free`, so the buffer
creator maintains the allocation lifecycle of the buffer's contents.

Fix Mac build without OpenSSL

Test pushing a file on-disk into a streaming filter that compresses
it into the ODB, and inflates it back into the working directory.

Let the filters use the checkout data's temporary buffer, instead
of having to allocate new buffers each time.

Use the new streaming filter API during checkout.

Migrate the `git_filter_list_apply_*` functions over to using the
new filter streams.

If OpenSSL is disabled on Mac the SHA1 implementation goes
to the CommonCrypto from the system. In this case we should
not include the generic hash impl. Otherwise there would be
duplicated impls which fail the build.

Add structures and preliminary functions to take a buffer, file or
blob and write the contents in chunks through an arbitrary number
of chained filters, finally writing into a user-provided function
accept the contents.

Remove extra semicolon outside of a function