tutorial: expanded discussion of commit history

tutorial: expanded discussion of commit history

Expand the history-browsing section of the tutorial a bit, in part to
address Junio's suggestion that we mention "git grep" and Linus's
complaint that people are missing the flexibility of the commandline
interfaces for selecting commits.

This reads a little more like a collection of examples than a
"tutorial", but maybe that's what people need at this point.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>