reachable: report precise timestamps from objects in cruft packs

When generating a cruft pack, the caller within pack-objects will want
to know the precise timestamps of cruft objects (i.e., their
corresponding values in the .mtimes table) rather than the mtime of the
cruft pack itself.

Teach add_recent_packed() to lookup each object's precise mtime from the
.mtimes file if one exists (indicated by the is_cruft bit on the
packed_git structure).

A couple of small things worth noting here:

  - load_pack_mtimes() needs to be called before asking for
    nth_packed_mtime(), and that call is done lazily here. That function
    exits early if the .mtimes file has already been opened and parsed,
    so only the first call is slow.

  - Checking the is_cruft bit can be done without any extra work on the
    caller's behalf, since it is set up for us automatically as a
    side-effect of calling add_packed_git() (just like the 'pack_keep'
    and 'pack_promisor' bits).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reachable.c

@@ -13,6 +13,7 @@
 #include "worktree.h"
 #include "object-store.h"
 #include "pack-bitmap.h"
+#include "pack-mtimes.h"
 
 struct connectivity_progress {
 	struct progress *progress;
@@ -155,6 +156,7 @@ static int add_recent_packed(const struct object_id *oid,
 			     void *data)
 {
 	struct object *obj;
+	timestamp_t mtime = p->mtime;
 
 	if (!want_recent_object(data, oid))
 		return 0;
@@ -163,7 +165,12 @@ static int add_recent_packed(const struct object_id *oid,
 
 	if (obj && obj->flags & SEEN)
 		return 0;
-	add_recent_object(oid, p, nth_packed_object_offset(p, pos), p->mtime, data);
+	if (p->is_cruft) {
+		if (load_pack_mtimes(p) < 0)
+			die(_("could not load cruft pack .mtimes"));
+		mtime = nth_packed_mtime(p, pos);
+	}
+	add_recent_object(oid, p, nth_packed_object_offset(p, pos), mtime, data);
 	return 0;
 }