submodule: test updating a submodule w/ a path

Test that `git_submodule_update` can handle a submodule that is
freshly cloned and has a path differing from its name.

Author: Edward Thomson

tests/resources/submodule_with_path/.gitmodules

@@ -0,0 +1,3 @@
+[submodule "testrepo"]
+	path = lib/testrepo
+	url = ../testrepo.git

tests/resources/submodule_with_path/.gitted/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/master

tests/resources/submodule_with_path/.gitted/config

@@ -0,0 +1,8 @@
+[core]
+	repositoryformatversion = 0
+	filemode = false
+	bare = false
+	logallrefupdates = true
+	symlinks = false
+	ignorecase = true
+	hideDotFiles = dotGitOnly