Expand git-diff page

Added:
  * `—names-only` option.
  * `—summary` option.
Update git-log.md

adds the command for displaying a detailed git history of a particular file or folder
Added example of overwriting an existing link
add extraction of bzipped archive
Pages: chsh
Add ag (the_silver_searcher) page
Missing apostrophe

Missing apostrophe in monitor "node" processes
Pages: psql