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

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.

When we have an HTTP stream and have set the certificatre check
callback, we currently fail as we ask the unencrypted stream for its
certificate.