Minor clean up of release notes

* Consistently use 'Git' when referring to the system (not 'git').
  https://github.com/git/git/blob/v2.1.3/Documentation/CodingGuidelines#L483-L486

* Remove angle brackets from a few urls. This is pseudo-wikitext.
  And even as plain text, it only seems confusing. Most other
  urls here, and in HISTORY, don't have it either.

* Remove version numbers from the "External libraries" message.
  We generally announce upgrades in the "New features" section.
  Having to keep this section up to date as we upgrade libraries
  seems unnecessary.

Change-Id: I4a8a75193cf6723ae734067f5d91ffa75692b5fd