compat: make obshistory._getdifflines compatible with mercurial 4.1 On Mercurial version 4.1 and lower, patch.diff seems to returns an empty list when there is no change. Mercurial 4.2 returns None so we add a if for this case.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t