repo: Add archive/zlib-level option, drop default compression to 6

The gzip default is 6.  When I was writing this code, I chose 9 under
the assumption that for long-term archival, the extra compression was
worth it.

Turns out level 9 is really, really not worth it.  Here's run at level 9
compressing the current Fedora Atomic Host into archive:

```
ostree --repo=repo pull-local repo-build fedora-atomic/25/x86_64/docker-host
real    2m38.115s
user    2m31.210s
sys     0m3.114s
617M    repo
```

And here's the new default level of 6:

```
ostree --repo=repo pull-local repo-build fedora-atomic/25/x86_64/docker-host
real    0m53.712s
user    0m43.727s
sys     0m3.601s
619M    repo
619M    total
```

As you can see, we run almost *three times* faster, and we take up *less
than one percent* more space.

Conclusion: Using level 9 is dumb.  And here's a run at compression level 1:

```
ostree --repo=repo pull-local repo-build fedora-atomic/25/x86_64/docker-host
real    0m24.073s
user    0m17.574s
sys     0m2.636s
643M    repo
643M    total
```

I would argue actually many people would prefer even this for "devel" repos.
For production repos, you want static deltas anyways.  (However, perhaps
we should support a model where generating a delta involves re-compressing
fallback objects with a bit stronger compression level).

Anyways, let's make everyone's life better and switch the default to 6.

Closes: #671
Approved by: jlebon

src/libostree/ostree-core.c

@@ -453,6 +453,34 @@ header_and_input_to_stream (GVariant           *file_header,
   return TRUE;
 }
 
+gboolean
+_ostree_raw_file_to_archive_stream (GInputStream       *input,
+                                    GFileInfo          *file_info,
+                                    GVariant           *xattrs,
+                                    guint               compression_level,
+                                    GInputStream      **out_input,
+                                    GCancellable       *cancellable,
+                                    GError            **error)
+{
+  g_autoptr(GVariant) file_header = NULL;
+  g_autoptr(GInputStream) zlib_input = NULL;
+
+  file_header = _ostree_zlib_file_header_new (file_info, xattrs);
+  if (input != NULL)
+    {
+      g_autoptr(GConverter) zlib_compressor = NULL;
+
+      zlib_compressor = G_CONVERTER (g_zlib_compressor_new (G_ZLIB_COMPRESSOR_FORMAT_RAW, compression_level));
+      zlib_input = g_converter_input_stream_new (input, zlib_compressor);
+    }
+  return header_and_input_to_stream (file_header,
+                                     zlib_input,
+                                     out_input,
+                                     NULL,
+                                     cancellable,
+                                     error);
+}
+
 /**
  * ostree_raw_file_to_archive_z2_stream:
  * @input: File raw content stream
@@ -473,23 +501,9 @@ ostree_raw_file_to_archive_z2_stream (GInputStream       *input,
                                       GCancellable       *cancellable,
                                       GError            **error)
 {
-  g_autoptr(GVariant) file_header = NULL;
-  g_autoptr(GInputStream) zlib_input = NULL;
-
-  file_header = _ostree_zlib_file_header_new (file_info, xattrs);
-  if (input != NULL)
-    {
-      g_autoptr(GConverter) zlib_compressor = NULL;
-
-      zlib_compressor = G_CONVERTER (g_zlib_compressor_new (G_ZLIB_COMPRESSOR_FORMAT_RAW, 9));
-      zlib_input = g_converter_input_stream_new (input, zlib_compressor);
-    }
-  return header_and_input_to_stream (file_header,
-                                     zlib_input,
-                                     out_input,
-                                     NULL,
-                                     cancellable,
-                                     error);
+  return _ostree_raw_file_to_archive_stream (input, file_info, xattrs,
+                                             OSTREE_ARCHIVE_DEFAULT_COMPRESSION_LEVEL,
+                                             out_input, cancellable, error);
 }
 
 /**