sources: check if ostree object exists in cache

The generic ways of checking if an object is in the cache does not apply
for ostree as the internal structure of a repo is quite specific. Thus
we need to use the ostree executable to ask it to explore its repo for
us.

sources/org.osbuild.ostree

@@ -11,6 +11,7 @@ import os
 import sys
 import subprocess
 import uuid
+from osbuild.util.ostree import show
 
 from osbuild import sources
 
@@ -122,8 +123,12 @@ class OSTreeSource(sources.SourceService):
         ostree("config", "set", "repo.locking", "true", repo=self.repo)
 
     # pylint: disable=[no-self-use]
-    def exists(self, _checksum, _item):
-        return False
+    def exists(self, checksum, _desc):
+        try:
+            show(self.repo, checksum)
+        except RuntimeError:
+            return False
+        return True
 
 
 def main():