You can clone with
HTTPS or Subversion.
I had a 1.xx piwik.
For some reasons, I had to delete all my files and do a clean install.
After downloading, unzipping and settings the rights, I copied my 1.xx config files to m y new installation to start the upgrade process.
I got an error message on this file : /core/SettingsPiwik.php:304
Line 304 is : $firstLineOfGitHead = file(PIWIK_INCLUDE_PATH . '/.git/HEAD');
The error is that the file doesn't exist. It is not included in the zip and haven't checked if it should have been downloaded before.
Thanks for the report, this was fixed in git 6e0d9cd