Merge branch 'jk/pretty-empty-format'

"git log --pretty/format=" with an empty format string did not mean
the more obvious "No output whatsoever" but "Use default format",
which was counterintuitive.

* jk/pretty-empty-format:
  pretty: make empty userformats truly empty
  pretty: treat "--format=" as an empty userformat
  revision: drop useless string offset when parsing "--pretty"

combine-diff.c

@@ -1407,7 +1407,8 @@ void diff_tree_combined(const unsigned char *sha1,
 		show_log(rev);
 
 		if (rev->verbose_header && opt->output_format &&
-		    opt->output_format != DIFF_FORMAT_NO_OUTPUT)
+		    opt->output_format != DIFF_FORMAT_NO_OUTPUT &&
+		    !commit_format_is_empty(rev->commit_format))
 			printf("%s%c", diff_line_prefix(opt),
 			       opt->line_termination);
 	}