setup-msysgit: Also set https_proxy

The URLs for the cloned repositories are now HTTPS URLs, therefore we
need to use the proxy also for such URLs if a proxy is required for
cloning to work.
setup-msysgit: avoid using the dashed form

It has been deprecated for quite a while now...

Besides, it does not seem to work here:
ln: accessing `//installer-tmp/bin/git.exe': No such host or network path
ln: accessing `//installer-tmp/bin/git.exe': No such host or network path
...
Merge pull request #197 from nilleb/patch-1

Issue 196, work in progress