fix misuse of prefix_path()

When DEFAULT_GIT_TEMPLATE_DIR is specified as a relative path,
init-db made it relative to exec_path using prefix_path(), which
is wrong.  prefix_path() is about a file inside the work tree.
There was a similar misuse in config.c that takes relative
ETC_GITCONFIG path.

A convenience function prefix_filename() can concatenate two paths
to form a path that points at somewhere outside the work tree.
Use it in these codepaths instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-init-db.c

@@ -142,8 +142,7 @@ static void copy_templates(const char *git_dir, int len, const char *template_di
 		template_dir = DEFAULT_GIT_TEMPLATE_DIR;
 		if (!is_absolute_path(template_dir)) {
 			const char *exec_path = git_exec_path();
-			template_dir = prefix_path(exec_path, strlen(exec_path),
-						   template_dir);
+			template_dir = prefix_filename(exec_path, strlen(exec_path), template_dir);
 		}
 	}
 	strcpy(template_path, template_dir);

config.c

@@ -485,8 +485,9 @@ const char *git_etc_gitconfig(void)
 		if (!is_absolute_path(system_wide)) {
 			/* interpret path relative to exec-dir */
 			const char *exec_path = git_exec_path();
-			system_wide = prefix_path(exec_path, strlen(exec_path),
-						system_wide);
+			system_wide = strdup(prefix_filename(exec_path,
+							     strlen(exec_path),
+							     system_wide));
 		}
 	}
 	return system_wide;