Update progit.org links to git-scm.com

- Link to log man page also folded into one hosted on git-scm
- Expand "CC" into "Creative Commons" since it's the first time used
- Trimmed the Markdown ATX heading hashing to go with majority
  using the leading and not on both sides on same source page
Merge pull request #22 from randomecho/linkrot

Update progit.org links to git-scm.com