gitweb: Hyperlink target of symbolic link in "tree" view (if possible)

gitweb: Hyperlink target of symbolic link in "tree" view (if possible)

Make symbolic link target in "tree" view into hyperlink to generic
"object" view (as we don't know if the link target is file (blob) or
directory (tree), and if it exist at all).

Target of link is made into hyperlink when:
 * hash_base is provided (otherwise we cannot find hash
   of link target)
 * link is relative
 * in no place link goes out of root tree (top dir)

Full path of symlink target from the root dir is provided in the title
attribute of hyperlink.

Currently symbolic link name uses ordinary file style (hidden
hyperlink), while the hyperlink to symlink target uses default
hyperlink style, so it is underlined while link target which is not
made into hyperlink is not underlined.

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