gitweb: Refactor generation of shortlog, tags and heads body

gitweb: Refactor generation of shortlog, tags and heads body

Add git_shortlog_body, git_tags_body and git_heads_body to generate
table with shortlog, tags and heads respectively in git_summary and
git_shortlog, git_tags, git_heads respectively.

Better support for lightweight tags in git_read_refs; currently only
lightweight tag pointing to tag object is not resolved fully.

Shortlog, tags and heads body tables have proper class now (we could
use id instead of class).

Add support for showing full comment on mouseover to tags list when
comment is shortened, similar to how full title of commit was/is
shown on mouseover when title was shortened.  Changed layout of tags
table to better show lightweight tags.

Add showing which branch (head) is current branch (current head),
using "current_head" class (we could use id instead).

Corrected "</table\n>" and hit_header_div instead of git_header_div.

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