changing incorrect reference of tag to tags
replace visionmedia/git-extras to tj/git-extras

Although Github reserves the 'visionmedia' user space(a visit to there
will be redirected to 'tj'), it's better to
rename the links from visionmedia/git-extras to tj/git-extras
correct the '-m' argument for grep. Then regenralize git-extra

'-m' option is supported both in GNU grep and in BSD grep
GNU grep: <http://linux.die.net/man/1/grep>
BSD grep:
<https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/grep.1.html>
correct the format of git-rebase-patch.md. Now we can handle it with script

Also regeneralize all markdown documents.
Merge pull request #272 from spacewander/master

replace visionmedia/git-extras to tj/git-extras
Merge pull request #271 from bohnman/git-rename-tag-fix

tag to tags