gitweb: Improve "next" link in commitdiff view

gitweb: Improve "next" link in commitdiff view

Check if 'hp' (hash_parent) parameter to 'commitdiff' view is one of
'h' (hash) commit parents, i.e. if commitdiff is of the form
"<commit>^<n> <commit>", and mark it as such in the bottom part of
navigation bar. The "next" link in commitdiff view was introduced
in commit 151602df00b8e5c5b4a8193f59a94b85f9b5aebc

If 'hb' is n-th parent of 'h', show the following at the bottom
of navigation bar:
  (from parent n: _commit_)

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