patch 9.0.1234: the code style has to be checked manually

Problem:    The code style has to be checked manually.
Solution:   Add basic code style checks in a test.  Fix or avoid uncovered
            problems.
diff --git a/src/diff.c b/src/diff.c
index ec8c8f1..a155c5e 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1680,9 +1680,8 @@
     {
 	if (dio->dio_internal)
 	{
-	    if (line_idx >= dout->dout_ga.ga_len) {
+	    if (line_idx >= dout->dout_ga.ga_len)
 		break;      // did last line
-	    }
 	    hunk = ((diffhunk_T **)dout->dout_ga.ga_data)[line_idx++];
 	}
 	else