config::include: sanitize homedir

Sanitize the home directory to ensure that we do not accidentally locate
a file called `~/.nonexistentfile`.

tests/config/include.c

@@ -113,6 +113,7 @@ void test_config_include__missing_homedir(void)
 	git_config *cfg;
 	git_buf buf = GIT_BUF_INIT;
 
+	cl_git_pass(git_libgit2_opts(GIT_OPT_SET_SEARCH_PATH, GIT_CONFIG_LEVEL_GLOBAL, cl_fixture("config")));
 	cl_git_mkfile("including", "[include]\npath = ~/.nonexistentfile\n[foo]\nbar = baz");
 
 	giterr_clear();
@@ -123,6 +124,8 @@ void test_config_include__missing_homedir(void)
 
 	git_buf_free(&buf);
 	git_config_free(cfg);
+
+	cl_sandbox_set_search_path_defaults();
 }
 
 #define replicate10(s) s s s s s s s s s s