gitweb: A bit of code cleanup in git_blame()

gitweb: A bit of code cleanup in git_blame()

Among others, here are the highlights:

 * move variable declaration closer to the place it is set and used,
   if possible,

 * uniquify and simplify coding style a bit, which includes removing
   unnecessary '()'.

 * check type only if $hash was defined, as otherwise from the way
   git_get_hash_by_path() is called (and works), we know that it is
   a blob,

 * use modern calling convention for git-blame,

 * remove unused variable,

 * don't use implicit variables ($_),

 * add some comments

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>