gitweb: Change the name of diff to parent link in "commit" view to "diff

gitweb: Change the name of diff to parent link in "commit" view to "diff

Change the name of diff to parent (current commit to one of parents)
link in "commit" view (git_commit subroutine) from "commitdiff" to
"diff".  Let's leave "commitdiff" for equivalent of git-show, or
git-diff-tree with one revision, i.e. diff for a given commit to its
parent (parents).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>