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

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.

--- a/tests/artifacts/cache/big-file-churn.hg.md5	Sun Feb 24 19:56:51 2019 +0100
+++ b/tests/artifacts/cache/big-file-churn.hg.md5	Sun Feb 24 19:56:46 2019 +0100
@@ -1,1 +1,1 @@
-403f5c247ad7cf58d3fad247a6b5e731
+cb1e67ae6b17ba66dd455f9782692f4a

--- a/tests/artifacts/scripts/generate-churning-bundle.py	Sun Feb 24 19:56:51 2019 +0100
+++ b/tests/artifacts/scripts/generate-churning-bundle.py	Sun Feb 24 19:56:46 2019 +0100
@@ -119,7 +119,7 @@
                 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()