gitweb: Use @diff_opts, default ('M'), as git-diff and git-diff-tree paramete

gitweb: Use @diff_opts, default ('M'), as git-diff and git-diff-tree paramete

Added new global configuration variable @diff_opts, which holds
additional options (parameters) to git-diff and git-diff-tree, usually
dealing rename/copying detection.  Default value is '-M', taken from
git_commit subroutine.  Description of options and their approximate
cost by Junio C Hamano.

Changes:
* git_commitdiff, git_blobdiff and git_blobdiff_plain now use '-M'
  instead of '-M', '-C'
* git-diff now uses the same options as git-diff-tree
* git_comittdiff_plain now uses '-M' instead of '-B'
  and is now rename-aware
* git_rss uses now '-M' instead of ()

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