tests: add coverage for repo.changelog.findmissing() in test-git-interop.t

This at least does a basic test of the method. It's not
super-complete, but it's better than the nothing we'd otherwise have.

Differential Revision: https://phab.mercurial-scm.org/D8547

tests/test-git-interop.t

@@ -248,3 +248,5 b' node|shortest works correctly'
   $ hg log -r ae1ab744f95bfd5b07cf573baef98a778058537b --template "{shortest(node,1)}\n"
   ae
 
+This coveres changelog.findmissing()
+  $ hg merge --preview 3d9be8deba43