Documentation: add a SEE ALSO section for merge-base

For example, a person reading the merge-base man page might wonder
about the fastest way to check if one commit is an ancestor of
another (which would require rev-list).
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-merge-base.txt

@@ -96,6 +96,12 @@ Documentation
 --------------
 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
 
+See also
+--------
+linkgit:git-rev-list[1],
+linkgit:git-show-branch[1],
+linkgit:git-merge[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite