diff: move show_interdiff() from its own file to diff-lib

show_interdiff() is a relatively small function and not likely to grow
larger or more complicated. Rather than dedicating an entire source file
to it, relocate it to diff-lib.c which houses other "take two things and
compare them" functions meant to be re-used but not so low-level as to
reside in the core diff implementation.

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

diff-lib.c

@@ -571,3 +571,27 @@ int index_differs_from(struct repository *r,
 	object_array_clear(&rev.pending);
 	return (rev.diffopt.flags.has_changes != 0);
 }
+
+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);
+}