wendelin.core: It needs git at runtime for wendelin.core-dev

Because git is used in `python setup.py sdist`

    https://lab.nexedi.cn/nexedi/wendelin.core/blob/11d130d1/setup.py#L168

we need to make sure it is available at runtime (at time when sdist
happens), or else installation could fail.

Since zc.recipe.egg:develop now supports setting environment variables

    https://lab.nexedi.cn/nexedi/slapos.buildout/commit/93d35f5d
    https://lab.nexedi.cn/nexedi/slapos.buildout/commit/71078537 )

and we use the proper version of it in slapos stack:

    https://lab.nexedi.cn/nexedi/slapos/commit/d0807d90

we can just set environment with git being in PATH and fix it this way.

/cc @Tyagov, @kazuhiko, @jerome
/reviewed testbot(building ok), trustme

component/wendelin.core/buildout.cfg

@@ -15,6 +15,11 @@ egg = wendelin.core
 recipe = zc.recipe.egg:develop
 egg = wendelin.core
 setup = ${wendelin.core-repository-submoduleinit:location}
+environment = wendelin.core-dev-env
+
+[wendelin.core-dev-env]
+# wendelin.core-dev needs git to build
+PATH = ${git:location}/bin:%(PATH)s
 
 
 [wendelin.core-repository]