update git links
This commit updates the links in section 1.1 (version control):
- gitref.org -> git.github.io/git-reference (old domain is not supported anymore, see this commit from the git-reference repo)
- try.github.com -> learngitbranching.js.org (try.github.com was discontinued, "Learn Git Branching" is a similar interactive tool, see this discussion in the git website repo)
- change link protocol from http to https in all links from this section
The resulting PDF can be found here.