gitweb: New improved patchset view (WIP !!!)

gitweb: New improved patchset view (WIP !!!)

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.

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>)'.
* <file> hyperlinking should work also when <file> is originally
  quoted. For now we present filename quoted. This needed changes to
  parse_difftree_raw_line subroutine. And doesn't work: perhaps
  unquote is broken.
* 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 1px.

Work in progress: might not work (and actually doesn't work correctly)
for strange filenames, i.e. filenames contaning either metacharacters
or having TAB, LF, backslash or doublequote in them.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>