git-log.txt: add links to 'rev-list' and 'diff' docs

Add links to the documentation for `git rev-list` and `git diff` instead of simply mentioning them, to make it easier for readers to reach these documentation pages. Let's link to `git diff` as this is the porcelain command, and the rest of the family (`diff-index`, `diff-tree` and `diff-files`) are mentioned in the "Raw output format" section of the `git diff` documentation. Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>