Add a custom param to git_smart_subtransport_definition

WinHTTP for MinGW

note: use a git_buf to return the default namespace

examples: add remote example.

checkout: report correct invalid path

In case of a bad url or other error during the connection setup, we
close the stream via free.

fix changelog

mkdir-ext: Assume directories don't exist; fix all race cases

win32 resource: allow custom comments field in DLL

Allow merges of files (and trees) with whitespace problems/fixes

The smart transport has already take the payload param. For the
sub transport a payload param is useful for the implementer.

clar: update to 08f434d

Update tips fixes

Lower case the include directive of windows header

PERF: In MERGE, lazily compute is_binary

Initialize refs vector in git_remote_update_tips().

Otherwise, bailing out early when ls_to_vector() fails accesses
uninitialized memory.

The caller has otherwise no way to know how long the string will be
allocated or ability to free it.

This fixes #2944.

Add annotated versions of ref-modying functions

[WIP] Smarter pack-building

Don't ask for a stream's certificate unless it's encrypted

Fix git_submodule_sync writing URL to wrong key.

Since the Linux platform has a case sensitive file system, the header name should be lower case for cross compiling purposes. (On Linux, the mingw header is called ```windows.h```).

link against crypt32 for CertFreeCertificateContext

When the user has a certificate check callback set, we still have to
check whether the stream we're using is even capable of providing a
certificate.

In the case of an unencrypted certificate, do not ask for it from the
stream, and do not call the callback.