interdiff: teach show_interdiff() to indent interdiff

A future change will allow "git format-patch --interdiff=<prev> -1" to
insert an interdiff into the commentary section of the lone patch of a
1-patch series. However, to prevent the inserted interdiff from
confusing git-am, as well as human readers, it needs to be indented.
Therefore, teach show_interdiff() how to indent.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1086,7 +1086,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 
 	if (rev->idiff_oid1) {
 		fprintf_ln(rev->diffopt.file, "%s", rev->idiff_title);
-		show_interdiff(rev);
+		show_interdiff(rev, 0);
 	}
 }
 

interdiff.c

@@ -3,15 +3,26 @@
 #include "revision.h"
 #include "interdiff.h"
 
-void show_interdiff(struct rev_info *rev)
+static struct strbuf *idiff_prefix_cb(struct diff_options *opt, void *data)
+{
+	return data;
+}
+
+void show_interdiff(struct rev_info *rev, int indent)
 {
 	struct diff_options opts;
+	struct strbuf prefix = STRBUF_INIT;
 
 	memcpy(&opts, &rev->diffopt, sizeof(opts));
 	opts.output_format = DIFF_FORMAT_PATCH;
+	opts.output_prefix = idiff_prefix_cb;
+	strbuf_addchars(&prefix, ' ', indent);
+	opts.output_prefix_data = &prefix;
 	diff_setup_done(&opts);
 
 	diff_tree_oid(rev->idiff_oid1, rev->idiff_oid2, "", &opts);
 	diffcore_std(&opts);
 	diff_flush(&opts);
+
+	strbuf_release(&prefix);
 }

interdiff.h

@@ -3,6 +3,6 @@
 
 struct rev_info;
 
-void show_interdiff(struct rev_info *);
+void show_interdiff(struct rev_info *, int indent);
 
 #endif