gitweb: SHA-1 in commit log message links to "object" view

gitweb: SHA-1 in commit log message links to "object" view

Instead of checking if explicit SHA-1 in commit log message is sha1 of
commit and making link to "commit" view, make [fragment of] explicit
SHA-1 in commit log message link to "object" view.  While at it allow
to hyperlink also shortened SHA-1, from 8 characters up to full SHA-1,
instead of requiring full 40 characters of SHA-1.

This makes the following changes:

 * SHA-1 of objects which no longer exists, for example in commit
   cherry-picked from no longer existing temporary branch, or revert
   of commit in rebased branch, are no longer marked as such by not
   being made into hyperlink (and not having default hyperlink view:
   being underlined among others).  On the other hand it makes gitweb
   to not write error messages when object is not found to web serwer
   log; it also moves cost of getting type and SHA-1 validation to
   when link is clicked, and not only viewed.

 * SHA-1 of other objects: blobs, trees, tags are also hyperlinked
   and lead to appropriate view (although in the case of tags it is
   more natural to just use tag name).

 * You can put shortened SHA-1 of commit in the commit message, and it
   would be hyperlinked; it would be checked on clicking if abbrev is
   unique.

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