tests: add `revlogutils.deltas` module to doctests

The doctest in these module have been from `mercurial.revlog` but the module was
not added to the doctests.

Spotted by Yuya Nishihara.

tests/test-doctest.py

@@ -61,6 +61,7 b" testmod('mercurial.pathutil')"
 testmod('mercurial.parser')
 testmod('mercurial.pycompat')
 testmod('mercurial.revlog')
+testmod('mercurial.revlogutils.deltas')
 testmod('mercurial.revsetlang')
 testmod('mercurial.smartset')
 testmod('mercurial.store')