dev-vcs/gitstats: fix the live ebuild's repo URI, license, dependencies, docs

dev-vcs/gitstats: fix the live ebuild's repo URI, license, dependencies, docs

The following changes have been made:
- eapi bump to EAPI 6
- inherit git-r3 instead of the obsolete git-2
- fix EGIT_REPO_URI
- fix LICENSE
  - the homepage has the following:
    "Both the code and the web site are licensed under GPLv2/GPLv3."
    So, use '|| ( GPL-2 GPL-3 )'
  - there's a bunled javascript file 'sortable.js' which is licensed under MIT
- add the missing build time dependency on perl
- install docs and manpage

Package-Manager: portage-2.2.26
Acked-by: Guilherme Amadio <amadio@gentoo.org>
Closes: https://github.com/gentoo/gentoo/pull/972