SVN
WordPress uses SVN internally such as for plugins repository. So, a plugin author must use SVN tools to upload a plugin and update the existing plugin/s in wp.org plugins repository. If you are new to SVN, WordPress docs team has a nice guide to get started with SVN and plugins repository. The best practices section is a gem. It is possible to update WP core using SVN. However, things have been changing. Now, you can host your plugin in Github and push it to wordpress.org plugins repo.
Failing automatic core security updates
If you have just started using version control (svn, git or hg) for your entire site and if the recent WordPress core security update hasn’t been applied, you are not alone. It is a known feature. To force WordPress to apply automatic updates, you may use the following code…
add_filter( 'automatic_updates_is_vcs_checkout', '__return_false', 1 );Alternatively, if you use git version control, you may change the location of .git directory. This method also helps to avoid backing up .git folder by mistake, when taking full-backups.
Git and Github
If you use git and Github for your WordPress development, then, WordPress core is available as a Github repo. Also, it is possible to install and update a plugin or theme that is hosted on Github (among others). Another interesting project around git and WP is gitium by Presslabs that supports CI (Continuous Integration).