Revert "run win64 in parallel again on appveyor"

This reverts commit a297d5c816ef26274eb5e48e953eba1c3f2a8094.

Sadly issue #7942 is still a problem
Bump libgit2 to 0.21.3
Bump Windows PortableGit to 1.9.5

Bump git to 1.8.5.6 for Mac binaries

add checksums for git