Add "--incremental" flag to git-pack-objects

It won't add an object that is already in a pack to the new pack.

pack-objects.c

@@ -5,7 +5,7 @@
 #include "pack.h"
 #include "csum-file.h"
 
-static const char pack_usage[] = "git-pack-objects [--window=N] [--depth=N] {--stdout | base-name} < object-list";
+static const char pack_usage[] = "git-pack-objects [--incremental] [--window=N] [--depth=N] {--stdout | base-name} < object-list";
 
 struct object_entry {
 	unsigned char sha1[20];
@@ -18,6 +18,7 @@ struct object_entry {
 	struct object_entry *delta;
 };
 
+static int incremental = 0;
 static struct object_entry **sorted_by_sha, **sorted_by_type;
 static struct object_entry *objects = NULL;
 static int nr_objects = 0, nr_alloc = 0;
@@ -192,6 +193,9 @@ static void add_object_entry(unsigned char *sha1, unsigned int hash)
 	unsigned int idx = nr_objects;
 	struct object_entry *entry;
 
+	if (incremental && has_sha1_pack(sha1))
+		return;
+
 	if (idx >= nr_alloc) {
 		unsigned int needed = (idx + 1024) * 3 / 2;
 		objects = xrealloc(objects, needed * sizeof(*entry));
@@ -387,6 +391,10 @@ int main(int argc, char **argv)
 		const char *arg = argv[i];
 
 		if (*arg == '-') {
+			if (!strcmp("--incremental", arg)) {
+				incremental = 1;
+				continue;
+			}
 			if (!strncmp("--window=", arg, 9)) {
 				char *end;
 				window = strtoul(arg+9, &end, 0);