storage: introduce a `revlog.reuse-external-delta-parent` config As pointed in c59987ab96b4, we had no simple way to get a client to not blindly reuse the delta parent from a bundle. Instead one had to rely on a side effect of the deprecated `format.generaldelta` configuration. We introduce an explicit `revlog.reuse-external-delta-parent` configuration option (default to True) to control this behavior. If the option is not set, `format.generaldelta` still control this behavior. To test the new option, we convert a couple of place where `generaldelta` have been used for its side effects.

--- a/mercurial/configitems.py	Sat Mar 02 09:41:17 2019 +0900
+++ b/mercurial/configitems.py	Wed Feb 27 10:49:25 2019 +0100
@@ -983,6 +983,9 @@
     default=True,
     alias=[('format', 'aggressivemergedeltas')],
 )
+coreconfigitem('storage', 'revlog.reuse-external-delta-parent',
+    default=None,
+)
 coreconfigitem('server', 'bookmarks-pushkey-compat',
     default=True,
 )