aufs: gen-patch: Fix Subject generation to skip SPDX-License-Identifier

(cherry picked from commit 809a0521f8)

I already did this on the sid branch but for some reason it didn't get
merged to master.

debian/changelog

@@ -1,3 +1,9 @@
+linux (4.16-1~exp2) UNRELEASED; urgency=medium
+
+  * aufs: gen-patch: Fix Subject generation to skip SPDX-License-Identifier
+
+ -- Ben Hutchings <ben@decadent.org.uk>  Sun, 08 Apr 2018 19:03:49 +0100
+
 linux (4.16-1~exp1) experimental; urgency=medium
 
   * New upstream release: https://kernelnewbies.org/Linux_4.16

debian/patches/features/all/aufs4/gen-patch

@@ -20,7 +20,7 @@ for patch in aufs4-{base,mmap,standalone}.patch; do
 	    git log --pretty='Date: %ad' HEAD -1 -- "$patch"
 	fi
 	echo -n 'Subject: '
-	head -1 "$aufs_dir/$patch"
+	grep -v '^SPDX-License-Identifier:' "$aufs_dir/$patch" | head -1
 	echo "$origin_line"
 	echo "$bug_line"
 	echo