Use Cow instead of String

src/cache.rs

@@ -1,21 +1,22 @@
 use crate::Error;
 use std::{
+    borrow::Cow,
     fs::{create_dir_all, File, OpenOptions},
     io::BufReader,
     path::Path,
 };
 
 /// Enum to indicate the state of the cache
-pub(crate) enum CacheState {
+pub(crate) enum CacheState<'a> {
     /// Current head and cached head are the same
     Current(u64),
     /// Cached head is older than current head
-    Old(Cache),
+    Old(Cache<'a>),
     /// No cache was found
     No,
 }
 
-impl CacheState {
+impl<'a> CacheState<'a> {
     pub(crate) fn read_from_file(path: impl AsRef<Path>, head: &str) -> Result<CacheState, Error> {
         if path.as_ref().exists() {
             let cache: Cache = serde_json::from_reader(BufReader::new(File::open(path)?))?;
@@ -29,7 +30,7 @@ impl CacheState {
         }
     }
 
-    pub(crate) fn calculate_new_cache(self, count: u64, head: String) -> Cache {
+    pub(crate) fn calculate_new_cache(self, count: u64, head: Cow<'a, str>) -> Cache {
         match self {
             CacheState::Old(mut cache) => {
                 cache.head = head;
@@ -42,12 +43,12 @@ impl CacheState {
 }
 
 #[derive(Serialize, Deserialize)]
-pub(crate) struct Cache {
-    pub head: String,
+pub(crate) struct Cache<'a> {
+    pub head: Cow<'a, str>,
     pub count: u64,
 }
 
-impl Cache {
+impl<'a> Cache<'a> {
     pub(crate) fn write_to_file(&self, path: impl AsRef<Path>) -> Result<(), Error> {
         create_dir_all(path.as_ref().parent().ok_or(Error::Internal)?)?;
         serde_json::to_writer(