checkpatch: don't emit unified-diff error for rename-only patches

I generated a patch with `git format-patch` which checkpatch thinks is
invalid:

    $ ./scripts/checkpatch.pl lpc-dt/0006-mfd-dt-Move-syscon-bindings-to-syscon-subdirectory.patch
    WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
     Documentation/devicetree/bindings/mfd/{ => syscon}/aspeed-scu.txt         | 0

    ERROR: Does not appear to be a unified-diff format patch

    total: 1 errors, 1 warnings, 0 lines checked

    NOTE: For some of the reported defects, checkpatch may be able to
          mechanically convert to the typical style using --fix or --fix-inplace.

    lpc-dt/0006-mfd-dt-Move-syscon-bindings-to-syscon-subdirectory.patch has style problems, please review.

    NOTE: If any of the errors are false positives, please report
          them to the maintainer, see CHECKPATCH in MAINTAINERS.

The patch in question was all renames with no edits, giving 100%
similarity and thus no diff markers.

Set '$is_patch = 1;' in the add/remove/rename detection to avoid
generating spurious warnings.

Link: http://lkml.kernel.org/r/20161205232224.22685-1-andrew@aj.id.au
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>