Add cross-references between docs for for-each-ref and show-ref

Add cross-references between the manpages for git-for-each-ref(1) and
git-show-ref(1).
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -227,6 +227,10 @@ Documentation
 -------------
 Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.
 
+SEE ALSO
+--------
+linkgit:git-show-ref[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite

Documentation/git-show-ref.txt

@@ -175,6 +175,7 @@ FILES
 
 SEE ALSO
 --------
+linkgit:git-for-each-ref[1],
 linkgit:git-ls-remote[1],
 linkgit:git-update-ref[1],
 linkgit:gitrepository-layout[5]