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>

scripts/checkpatch.pl

@@ -1279,6 +1279,11 @@ sub process {
 			}
 		}
 
+# Accept git diff extended headers as valid patches
+		if ($line =~ /^(?:rename|copy) (?:from|to) [\w\/\.\-]+\s*$/) {
+			$is_patch = 1;
+		}
+
 #check the patch for a signoff:
 		if ($line =~ /^\s*signed-off-by:/i) {
 			# This is a signoff, if ugly, so do not double report.