gitweb: Add "next" link to commitdiff view

gitweb: Add "next" link to commitdiff view

Add a kind of "next" view in the bottom part of navigation bar for
"commitdiff" view.

For commitdiff between two commits:
  (from: _commit_)
For commitdiff for one single parent commit:
  (parent: _commit_)
For commitdiff for one merge commit
  (merge: _commit_ _commit_ ...)
For commitdiff for root (parentless) commit
  (initial)
where _link_ denotes hyperlink. SHA1 is shortened to 7 characters on
display, everything is perhaps unnecessary esc_html on display.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>