Documentation: split off rev doc into include file

Documentation: split off rev doc into include file

Currently, the documentation for revisions and ranges sits in the
git-rev-parse man page, i.e. a plumbing man page, along with the
documentation of all rev-parse modes.

Split off the revisions and ranges section into an included file to
prepare for restructuring.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>