test: stabilize bundle generation for test-sparse-revlog.t

To reduce the instability in the bundle binary content, we force it to contains
delta against p1 in all cases.

In the previous changeset, we already stabilized the processing of the bundle.
So we don't see any output change in the test itself.

tests/artifacts/cache/big-file-churn.hg.md5

@@ -1,1 +1,1 b''
-403f5c247ad7cf58d3fad247a6b5e731
+cb1e67ae6b17ba66dd455f9782692f4a

tests/artifacts/scripts/generate-churning-bundle.py

@@ -119,7 +119,7 b' def run(target):'
                 hg('merge', 'min(head())')
             updatefile(FILENAME, idx)
             hg('commit', '--message', 'commit #%d' % idx)
-        hg('bundle', '--all', target)
+        hg('bundle', '--all', target, '--config', 'devel.bundle.delta=p1')
         with open(target, 'rb') as bundle:
             data = bundle.read()
             digest = hashlib.md5(data).hexdigest()