pretty: make empty userformats truly empty

If the user provides an empty format with "--format=", we
end up putting in extra whitespace that the user cannot
prevent. This comes from two places:

  1. If the format is missing a terminating newline, we add
     one automatically. This makes sense for --format=%h, but
     not for a truly empty format.

  2. We add an extra newline between the pretty-printed
     format and a diff or diffstat. If the format is empty,
     there's no point in doing so if there's nothing to
     separate.

With this patch, one can get a diff with no other cruft out
of "diff-tree --format= $commit".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1397,7 +1397,8 @@ 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);
 	}

commit.h

@@ -159,6 +159,7 @@ extern void get_commit_format(const char *arg, struct rev_info *);
 extern const char *format_subject(struct strbuf *sb, const char *msg,
 				  const char *line_separator);
 extern void userformat_find_requirements(const char *fmt, struct userformat_want *w);
+extern int commit_format_is_empty(enum cmit_fmt);
 extern void format_commit_message(const struct commit *commit,
 				  const char *format, struct strbuf *sb,
 				  const struct pretty_print_context *context);

log-tree.c

@@ -649,7 +649,7 @@ void show_log(struct rev_info *opt)
 		graph_show_commit_msg(opt->graph, &msgbuf);
 	else
 		fwrite(msgbuf.buf, sizeof(char), msgbuf.len, stdout);
-	if (opt->use_terminator) {
+	if (opt->use_terminator && !commit_format_is_empty(opt->commit_format)) {
 		if (!opt->missing_newline)
 			graph_show_padding(opt->graph);
 		putchar(opt->diffopt.line_termination);
@@ -676,7 +676,8 @@ int log_tree_diff_flush(struct rev_info *opt)
 		show_log(opt);
 		if ((opt->diffopt.output_format & ~DIFF_FORMAT_NO_OUTPUT) &&
 		    opt->verbose_header &&
-		    opt->commit_format != CMIT_FMT_ONELINE) {
+		    opt->commit_format != CMIT_FMT_ONELINE &&
+		    !commit_format_is_empty(opt->commit_format)) {
 			/*
 			 * When showing a verbose header (i.e. log message),
 			 * and not in --pretty=oneline format, we would want

pretty.c

@@ -24,6 +24,11 @@ static size_t commit_formats_len;
 static size_t commit_formats_alloc;
 static struct cmt_fmt_map *find_commit_format(const char *sought);
 
+int commit_format_is_empty(enum cmit_fmt fmt)
+{
+	return fmt == CMIT_FMT_USERFORMAT && !*user_format;
+}
+
 static void save_user_format(struct rev_info *rev, const char *cp, int is_tformat)
 {
 	free(user_format);