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
Expand git-diff page

Added:
  * `—names-only` option.
  * `—summary` option.
Merge pull request #184 from mateu-aguilo-bosch/git-diff-name-only

Expand git-diff page
Merge pull request #179 from Dahie/patch-2

Update git-log.md
Merge pull request #180 from andreassimon/patch-1

Added example of overwriting an existing link
add extraction of bzipped archive
Merge pull request #189 from Leandros/patch-1

add extraction of bzipped archive
Pages: chsh
Merge pull request #192 from Like-all/master

Pages: chsh
Add ag (the_silver_searcher) page
Merge pull request #194 from tsigo/rs-ag

Add ag (the_silver_searcher) page
Missing apostrophe

Missing apostrophe in monitor "node" processes
Merge pull request #195 from sanusart/patch-1

Missing apostrophe
Pages: ab
Fix ab page to conform common standards
Merge pull request #197 from igorshubovych/master

Pages: ab
Pages: psql