checkpatch: consider git extended headers valid patches

Renames look like this with git-diff(1) when diff.renames = true is set:

  diff --git a/a b/b
  similarity index 100%
  rename from a
  rename to b

This raises the "Does not appear to be a unified-diff format patch"
error because checkpatch.pl only considers a diff valid if it contains
at least one "@@" hunk.

This patch accepts renames and copies too so that checkpatch.pl exits
successfully when a diff only renames/copies files.  The git diff
extended header format is described on the git-diff(1) man page.

Reported-by: Colin Lord <clord@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1468576014-28788-1-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>