Fix changing git rev cache

src/cook/fetch.rs

@@ -238,14 +238,18 @@ pub fn fetch(recipe: &CookRecipe, check_source: bool, logger: &PtyOut) -> Result
                 run_command(command, logger)?;
 
                 let (head_rev, detached_rev) = get_git_head_rev(&source_dir)?;
-                if detached_rev {
-                    if let Some(rev) = rev
+                if rev.is_some() {
+                    if !detached_rev {
+                        false
+                    } else if let Some(rev) = rev
                         && let Ok(exp_rev) = get_git_tag_rev(&source_dir, &rev)
                     {
                         exp_rev == head_rev
                     } else {
                         false
                     }
+                } else if detached_rev {
+                    false
                 } else {
                     let (_, remote_branch, remote_name, remote_url) =
                         get_git_remote_tracking(&source_dir)?;

src/cook/fs.rs

@@ -298,6 +298,7 @@ pub fn get_git_tag_rev(dir: &PathBuf, tag: &str) -> Result<String> {
 }
 
 pub fn get_git_ref_entry(dir: &PathBuf, entry: &str) -> Result<String> {
+    // https://git-scm.com/book/en/v2/Git-Internals-Maintenance-and-Data-Recovery
     let git_refs = dir.join(".git/packed-refs");
     let refs_str = read_to_string(&git_refs)?;
     for line in refs_str.lines() {