SCRIPTS: backport: fix the master branch detection

The condition was inverted. When the branch was the master, it was
harmless because it caused an extra "checkout master", but when it
was not the master, the commit could be applied to the wrong branch
and it could even possibly not match the name to stop on.

scripts/backport

@@ -67,7 +67,7 @@ check_clean() {
 
 # verifies that HEAD is the master
 check_master() {
-	test "$(git rev-parse --verify -q HEAD 2>&1)" != "$(git rev-parse --verify -q master 2>&1)"
+	test "$(git rev-parse --verify -q HEAD 2>&1)" = "$(git rev-parse --verify -q master 2>&1)"
 }
 
 # tries to switch to the master branch, only if the current one is clean. Dies on failure.