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>

1 files changed, 12 insertions, 1 deletions

diff --git a/interdiff.c b/interdiff.c
index d0fac10..c81d680 100644
--- a/interdiff.c
+++ b/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);
 }