Replace "gitweb diff header" with its full sha1 of blobs and replace
it by "git diff" header and extended diff header. Change also somewhat
highlighting of diffs.

Added `file_type_long' subroutine to convert file mode in octal to
file type description (only for file modes which used by git).

Changes:
* "gitweb diff header" which looked for example like below:
    file:_<sha1 before>_ -> file:_<sha1 after>_
  where 'file' is file type and '<sha1>' is full sha1 of blob is
  changed to
    diff --git _a/<file before>_ _b/<file after>_
  In both cases links are visible and use default link style. If file
  is added, a/<file> is not hyperlinked. If file is deleted, b/<file>
  is not hyperlinked.
* there is added "extended diff header", with <path> and <hash>
  hyperlinked (and <hash> shortened to 7 characters), and <mode>
  explained: '<mode>' is extended to '<mode> (<file type description>)',
  where added text is slightly lighter to easy distinguish that it
  was added (and it is difference from git-diff output).
* from-file/to-file two-line header lines have slightly darker color
  than removed/added lines.
* chunk header has now delicate line above for easier finding chunk
  boundary, and top margin of 2px, both barely visible.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>