diff --git a/diff.c b/diff.c
index fa78fc189c..bb816d9cba 100644
--- a/diff.c
+++ b/diff.c
@@ -4449,7 +4449,7 @@ static void patch_id_consume(void *priv, char *line, unsigned long len)
 }
 
 /* returns 0 upon success, and writes result into sha1 */
-static int diff_get_patch_id(struct diff_options *options, unsigned char *sha1)
+static int diff_get_patch_id(struct diff_options *options, unsigned char *sha1, int diff_header_only)
 {
 	struct diff_queue_struct *q = &diff_queued_diff;
 	int i;
@@ -4484,9 +4484,6 @@ static int diff_get_patch_id(struct diff_options *options, unsigned char *sha1)
 
 		diff_fill_sha1_info(p->one);
 		diff_fill_sha1_info(p->two);
-		if (fill_mmfile(&mf1, p->one) < 0 ||
-				fill_mmfile(&mf2, p->two) < 0)
-			return error("unable to read files to diff");
 
 		len1 = remove_space(p->one->path, strlen(p->one->path));
 		len2 = remove_space(p->two->path, strlen(p->two->path));
@@ -4521,6 +4518,13 @@ static int diff_get_patch_id(struct diff_options *options, unsigned char *sha1)
 					len2, p->two->path);
 		git_SHA1_Update(&ctx, buffer, len1);
 
+		if (diff_header_only)
+			continue;
+
+		if (fill_mmfile(&mf1, p->one) < 0 ||
+		    fill_mmfile(&mf2, p->two) < 0)
+			return error("unable to read files to diff");
+
 		if (diff_filespec_is_binary(p->one) ||
 		    diff_filespec_is_binary(p->two)) {
 			git_SHA1_Update(&ctx, sha1_to_hex(p->one->sha1), 40);
@@ -4541,11 +4545,11 @@ static int diff_get_patch_id(struct diff_options *options, unsigned char *sha1)
 	return 0;
 }
 
-int diff_flush_patch_id(struct diff_options *options, unsigned char *sha1)
+int diff_flush_patch_id(struct diff_options *options, unsigned char *sha1, int diff_header_only)
 {
 	struct diff_queue_struct *q = &diff_queued_diff;
 	int i;
-	int result = diff_get_patch_id(options, sha1);
+	int result = diff_get_patch_id(options, sha1, diff_header_only);
 
 	for (i = 0; i < q->nr; i++)
 		diff_free_filepair(q->queue[i]);
diff --git a/diff.h b/diff.h
index 125447be09..7883729edf 100644
--- a/diff.h
+++ b/diff.h
@@ -342,7 +342,7 @@ extern int run_diff_files(struct rev_info *revs, unsigned int option);
 extern int run_diff_index(struct rev_info *revs, int cached);
 
 extern int do_diff_cache(const unsigned char *, struct diff_options *);
-extern int diff_flush_patch_id(struct diff_options *, unsigned char *);
+extern int diff_flush_patch_id(struct diff_options *, unsigned char *, int);
 
 extern int diff_result_code(struct diff_options *, int);
 
diff --git a/patch-ids.c b/patch-ids.c
index bafaae2a6a..69a14a3b50 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -13,7 +13,7 @@ int commit_patch_id(struct commit *commit, struct diff_options *options,
 	else
 		diff_root_tree_sha1(commit->object.oid.hash, "", options);
 	diffcore_std(options);
-	return diff_flush_patch_id(options, sha1);
+	return diff_flush_patch_id(options, sha1, 0);
 }
 
 static int patch_id_cmp(struct patch_id *a,