perf: Lazy load files from directory (#335)

Changes context to use `once_cell` to lazily evaluate directory listing on first use.

tests/testsuite/git_branch.rs

@@ -1,5 +1,4 @@
 use ansi_term::Color;
-use git2::Repository;
 use std::io;
 use std::process::Command;
 

tests/testsuite/git_status.rs

@@ -1,5 +1,4 @@
 use ansi_term::Color;
-use git2::Repository;
 use std::fs::{self, File};
 use std::io;
 use std::process::Command;

tests/testsuite/time.rs

@@ -1,8 +1,4 @@
-use ansi_term::Color;
-use std::fs;
 use std::io;
-use std::path::Path;
-use tempfile::TempDir;
 
 use crate::common::{self, TestCommand};