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-libarchive.c

@@ -480,6 +480,12 @@ file_to_archive_entry_common (GFile         *root,
   g_autoptr(GVariant) xattrs = NULL;
   time_t ts = (time_t) opts->timestamp_secs;
 
+  if (opts->path_prefix && opts->path_prefix[0])
+    {
+      g_autofree char *old_pathstr = pathstr;
+      pathstr = g_strconcat (opts->path_prefix, old_pathstr, NULL);
+    }
+
   if (pathstr == NULL || !pathstr[0])
     {
       g_free (pathstr);

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