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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

Documentation/git-log.txt

@@ -15,9 +15,9 @@ DESCRIPTION
 -----------
 Shows the commit logs.
 
-The command takes options applicable to the `git rev-list`
+The command takes options applicable to the linkgit:git-rev-list[1]
 command to control what is shown and how, and options applicable to
-the `git diff-*` commands to control how the changes
+the linkgit:git-diff[1] command to control how the changes
 each commit introduces are shown.