ci: use git-sdk-64-minimal build artifact

ci: use git-sdk-64-minimal build artifact

Instead of a shallow fetch followed by a sparse checkout, we are
better off by using a separate, dedicated Pipeline that bundles
the SDK as a build artifact, and then consuming that build artifact
here.

In fact, since this artifact will be used a lot, we spent substantial
time on figuring out a minimal subset of the Git for Windows SDK, just
enough to build and test Git. The result is a size reduction from around
1GB (compressed) to around 55MB (compressed). This also comes with the
change where we now call `usr\bin\bash.exe` directly, as `git-cmd.exe`
is not included in the minimal SDK.

That reduces the time to initialize Git for Windows' SDK from anywhere
between 2m30s-7m to a little over 1m.

Note: in theory, we could also use the DownloadBuildArtifacts@0 task
here. However, restricted permissions that are in effect when building
from forks would let this fail for PR builds, defeating the whole
purpose of the Azure Pipelines support for git.git.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>