tests: use a dummy homedir

Use a dummy home directory for tests to avoid user data leaking into
test execution.

tests/clar/clar_libgit2.c

@@ -623,6 +623,24 @@ void cl_fake_globalconfig(git_str *out)
 	git_str_dispose(&path);
 }
 
+void cl_sandbox_set_homedir(const char *home)
+{
+	git_str path = GIT_STR_INIT;
+
+	if (home) {
+		git_libgit2_opts(GIT_OPT_SET_HOMEDIR, home);
+	} else {
+		git_str_joinpath(&path, clar_sandbox_path(), "__home");
+
+		if (!git_fs_path_exists(path.ptr))
+			cl_must_pass(p_mkdir(path.ptr, 0777));
+
+		git_libgit2_opts(GIT_OPT_SET_HOMEDIR, path.ptr);
+	}
+
+	git_str_dispose(&path);
+}
+
 void cl_sandbox_set_search_path_defaults(void)
 {
 	git_str path = GIT_STR_INIT;

tests/clar/clar_libgit2.h

@@ -249,6 +249,7 @@ void cl_fake_homedir_cleanup(void *);
 void cl_fake_globalconfig(git_str *);
 void cl_fake_globalconfig_cleanup(void *);
 
+void cl_sandbox_set_homedir(const char *);
 void cl_sandbox_set_search_path_defaults(void);
 void cl_sandbox_disable_ownership_validation(void);
 

tests/clar/main.c

@@ -25,6 +25,7 @@ int main(int argc, char *argv[])
 	}
 
 	cl_global_trace_register();
+	cl_sandbox_set_homedir(getenv("CLAR_HOMEDIR"));
 	cl_sandbox_set_search_path_defaults();
 	cl_sandbox_disable_ownership_validation();