gitweb: New improved patchset view

gitweb: New improved patchset view

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: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>