core: Use mmap() with O_NOATIME if merely reading archive-z2 entries

Previously we'd open(path, O_NOATIME) and do a series of small read()
calls to just parse the header.  I think this will trigger kernel readahead
into the compressed portion, but we don't care about that.

This should be more efficient.

src/libostree/ostree-core.c

@@ -540,10 +540,11 @@ ostree_content_file_parse (gboolean                compressed,
   struct stat stbuf;
   ot_lobj GInputStream *file_input = NULL;
   ot_lobj GInputStream *ret_input = NULL;
-  ot_lobj GFileInfo *content_file_info = NULL;
   ot_lobj GFileInfo *ret_file_info = NULL;
   ot_lvariant GVariant *ret_xattrs = NULL;
 
+  if (out_input)
+    {
       file_input = (GInputStream*)gs_file_read_noatime (content_path, cancellable, error);
       if (!file_input)
         goto out;
@@ -555,6 +556,22 @@ ostree_content_file_parse (gboolean                compressed,
         }
 
       length = stbuf.st_size;
+    }
+  else
+    {
+      GMappedFile *mmaped;
+      GBytes *bytes;
+
+      mmaped = gs_file_map_noatime (content_path, cancellable, error);
+      if (!mmaped)
+        goto out;
+      bytes = g_mapped_file_get_bytes (mmaped);
+      g_mapped_file_unref (mmaped);
+      mmaped = NULL;
+      file_input = g_memory_input_stream_new_from_bytes (bytes);
+      length = g_bytes_get_size (bytes);
+      g_bytes_unref (bytes);
+    }
 
   if (!ostree_content_stream_parse (compressed, file_input, length, trusted,
                                     out_input ? &ret_input : NULL,