gitweb: Add "next" link to commit view

gitweb: Add "next" link to commit view

Add a kind of "next" view in the bottom part of navigation bar for
"commit" view, similar to what was added for "commitdiff" view in
commit 151602df00b8e5c5b4a8193f59a94b85f9b5aebc
  'gitweb: Add "next" link to commitdiff view'

For "commit" view for single parent commit:
  (parent: _commit_)
For "commit" view for merge (multi-parent) commit:
  (merge: _commit_ _commit_ ...)
For "commit" view for root (parentless) commit
  (initial)
where _link_ denotes hyperlink.  SHA1 of commit is shortened
to 7 characters on display.

While at it, remove leftovers from commit cae1862a by Petr Baudis:
  'gitweb: More per-view navigation bar links'
namely the "blame" link if there exist $file_name and commit has a
parent; it was added in git_commit probably by mistake.  The rest
of what mentioned commit added for git_commit was removed in
commit 6e0e92fda893311ff5af91836e5007bf6bbd4a21 by Luben Tuikov:
  'gitweb: Do not print "log" and "shortlog" redundantly in commit view'
(which should have probably removed also this "blame" link removed now).

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