ostree export: Add --prefix option

This lets you set a prefix for the resulting archive patsh.
Especially useful in combination with --subpath, for instance
--subpath=subdir --prefix=subdir to extract just subdir.

Closes: #265
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -610,7 +610,10 @@ typedef struct {
   guint64 timestamp_secs;
 
   guint unused_uint[8];
-  gpointer unused_ptrs[8];
+
+  char *path_prefix;
+
+  gpointer unused_ptrs[7];
 } OstreeRepoExportArchiveOptions;
 
 _OSTREE_PUBLIC