gitweb cleanup: Move @diff_opts declaration earlier

gitweb cleanup: Move @diff_opts declaration earlier

Move @diff_opts declaration earlier, so that all gitweb options are
together (and not separated by %feature hash and some subroutines),
with the exception of $GITWEB_CONFIG which must be after all option
variables including %feature hash.

While at it, in the moved comment, note that diff option '-C' implies
'-M', instead of suggesting that '-M', '-C' is required.

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