tools: moveconfig: show result of header cleaning in unified diff

tools: moveconfig: show result of header cleaning in unified diff

The header cleanup feature of this tool now removes empty ifdef's,
successive blank lines as well as moved option defines.  So, we
want to see a little more context to check which lines were deleted.

It is true that we can see it by "git diff", but it would not work
in the --dry-run mode.  So, here, this commit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>