Documentation: fix documentation AsciiDoc links for external urls

Documentation: fix documentation AsciiDoc links for external urls

Turns out that putting 'link:' before the 'http' is actually superfluous
in AsciiDoc, as there's already a predefined macro to handle it.

"http, https, [etc] URLs are rendered using predefined inline macros."
http://www.methods.co.nz/asciidoc/userguide.html#_urls

"Hypertext links to files on the local file system are specified
using the link inline macro."
http://www.methods.co.nz/asciidoc/userguide.html#_linking_to_local_documents

Despite being superfluous, the reference implementation of AsciiDoc
tolerates the extra 'link:' and silently removes it, giving a functioning
link in the generated HTML. However, AsciiDoctor (the Ruby implementation
of AsciiDoc used to render the http://git-scm.com/ site) does /not/ have
this behaviour, and so generates broken links, as can be seen here:

http://git-scm.com/docs/git-cvsimport (links to cvs2git & parsecvs)
http://git-scm.com/docs/git-filter-branch (link to The BFG)

It's worth noting that after this change, the html generated by 'make html'
in the git project is identical, and all links still work.

Signed-off-by: Roberto Tyley <roberto.tyley@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>