Tutorial updates.

 - Use "working tree", "object name", "repository" as the canonical
   term consistenly.

 - Start formatting tutorial with asciidoc.

 - Mention shared repository style of cooperation.

 - Update with some usability enhancements recently made, such as
   the "-m" flag to the "git commit" command.

Signed-off-by: Junio C Hamano <junkio@cox.net>