Documentation: link to gitrevisions rather than git-rev-parse

Documentation: link to gitrevisions rather than git-rev-parse

Currently, whenever we need documentation for revisions and ranges, we
link to the git-rev-parse man page, i.e. a plumbing man page, which has
this along with the documentation of all rev-parse modes.

Link to the new gitrevisions man page instead in all cases except
- when the actual git-rev-parse command is referred to or
- in very technical context (git-send-pack).

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