ZODB: Switch persistent to git checkout

We'll need persistent as git checkout in the next patch when organizing testing
of ZODB stack, because persistent tests want to discover in-tree files that are
not present in persistent egg when it is installed in non-development mode:

https://erp5.nexedi.net/test_result_module/20201123-3F859E35/7
(look for "AssertionError: could not find my setup.py")

https://github.com/zopefoundation/persistent/blob/4.6.4-0-g7ed95cf/persistent/tests/test_docs.py#L37-L43

component/ZODB/buildout.cfg

@@ -68,7 +68,6 @@ depends = ${persistent:egg} ${BTrees:egg}
 # eggs versions that are used by all ZODB{4,5}.
 egg_versions =
   BTrees = 4.5.1
-  persistent = 4.6.4
   zodbpickle = 1.0.4
 
   # Provide ZODB3 for those eggs that still care about ZODB3 compatibility -
@@ -154,6 +153,14 @@ egg = ${:_buildout_section_name_}
 setup-eggs = ${persistent:egg}
 
 [persistent]
-recipe = zc.recipe.egg:custom
+recipe = zc.recipe.egg:develop
 egg = ${:_buildout_section_name_}
 setup-eggs = ${python-cffi:egg}
+setup = ${persistent-repository:location}
+
+[persistent-repository]
+<= _ZODB-git-repository
+location = ${buildout:parts-directory}/persistent
+repository = https://github.com/zopefoundation/persistent.git
+branch = master
+revision = 4.6.4-0-g7ed95cf