wendelin.core: Use SlapOS-provided Git, where appropriate

Quoting @jerome:

    @kirr, we usually extends component/git/buildout.cfg and use
    ${git:location}/bin/git instead of git, in case git is not always in the
    path, especially in webrunner.

component/wendelin.core/buildout.cfg

 [buildout]
+extends = ../git/buildout.cfg
 parts =
   wendelin.core
 
@@ -20,13 +21,14 @@ recipe  = slapos.recipe.build:gitclone
 repository = https://lab.nexedi.cn/nexedi/wendelin.core.git
 # dir is pretty name as top-level -dev recipe
 location = ${buildout:parts-directory}/wendelin.core-dev
+git-executable = ${git:location}/bin/git
 
 
 # TODO add `git clone --recursive` to slapos.recipe.build:gitclone
 #      and this way merge this -submoduleinit into -repository part
 [wendelin.core-repository-submoduleinit]
 recipe = plone.recipe.command
-command = cd "${wendelin.core-repository:location}" && git submodule update --init
+command = cd "${wendelin.core-repository:location}" && ${git:location}/bin/git submodule update --init
 stop-on-error = true
 # propagate location of main repo
 location= ${wendelin.core-repository:location}