Update autobuild utility for git workflow

[historic_bruno]

Currently the Windows autobuilder works with our SVN repository (see BuildServerSetup). As we're transitioning to git, we need it to work with a git repo instead.

• support multiple git branches, both the "master" and various feature branches, so e.g. artists can easily test new features
• commit a file for the relevant branch, that can be tested by an updater utility, and contains paths and checksums of each file that was built
• the built files will be uploaded to a web server

See also #1814 and #1816.