fix: speedup HEAD detection in is_git()

Previously it would always check with a `store` which has a fuzzy search
right now. Instead, we check once ourselves before trying to read HEAD
for validation.

gix-discover/src/is.rs

@@ -98,6 +98,11 @@ pub fn git(git_dir: impl AsRef<Path>) -> Result<crate::repository::Kind, crate::
     };
 
     {
+        // Fast-path: avoid doing the complete search if HEAD is already not there.
+        // TODO(reftable): use a ref-store to lookup HEAD if ref-tables should be supported, or detect ref-tables beforehand.
+        if !dot_git.join("HEAD").exists() {
+            return Err(crate::is_git::Error::MissingHead);
+        }
         // We expect to be able to parse any ref-hash, so we shouldn't have to know the repos hash here.
         // With ref-table, the has is probably stored as part of the ref-db itself, so we can handle it from there.
         // In other words, it's important not to fail on detached heads here because we guessed the hash kind wrongly.

gix-discover/src/lib.rs

@@ -23,6 +23,8 @@ pub mod is_git {
     pub enum Error {
         #[error("Could not find a valid HEAD reference")]
         FindHeadRef(#[from] gix_ref::file::find::existing::Error),
+        #[error("Missing HEAD at '.git/HEAD'")]
+        MissingHead,
         #[error("Expected HEAD at '.git/HEAD', got '.git/{}'", .name)]
         MisplacedHead { name: bstr::BString },
         #[error("Expected an objects directory at '{}'", .missing.display())]