get_maintainer.pl: find maintainers for removed files

For removed files, get_maintainer.pl doesn't find any maintainers (besides
the default linux-kernel@vger.kernel.org), as it only looks at the "+++"
lines, which are "/dev/null" for removals.  Fix this by extending the
parsing to the "---" lines.

E.g. for the two line test patch below the real score maintainers will now
be found:

    --- a/arch/score/include/asm/dma-mapping.h
    +++ /dev/null

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>