Don't dereference top-level regular files

Since this yielded an empty directory as far back as Nix 2.3, we don't
really need special handling for executables vs non-executables.

src/libfetchers/git-utils.cc

@@ -491,14 +491,13 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
     {
         auto oid = hashToOID(oid_);
 
-        /* If the root directory contains  */
         auto _tree = lookupObject(*this, oid, GIT_OBJECT_TREE);
         auto tree = (const git_tree *) &*_tree;
 
         if (git_tree_entrycount(tree) == 1) {
             auto entry = git_tree_entry_byindex(tree, 0);
             auto mode = git_tree_entry_filemode(entry);
-            if (mode == GIT_FILEMODE_BLOB || mode == GIT_FILEMODE_TREE)
+            if (mode == GIT_FILEMODE_TREE)
                 oid = *git_tree_entry_id(entry);
         }
 

src/libfetchers/git-utils.hh

@@ -101,12 +101,7 @@ struct GitRepo
 
     /**
      * If the specified Git object is a directory with a single entry
-     * that is a directory or a non-executable regular file, return
-     * the ID of that object.
-     *
-     * Note: We don't do this for executable files because they don't
-     * have a tree hash in the Git object model that distinguishes
-     * them from non-executable files.
+     * that is a directory, return the ID of that object.
      */
     virtual Hash dereferenceSingletonDirectory(const Hash & oid) = 0;
 };

tests/functional/tarball.sh

@@ -92,15 +92,7 @@ path="$(nix flake prefetch --json "tarball+file://$TEST_ROOT/tar.tar" | jq -r .s
 [[ -d "$path/foo" ]]
 [[ -d "$path/bar" ]]
 
-# Test a tarball that has a single non-executable regular file.
-rm -rf "$TEST_ROOT/tar_root"
-mkdir -p "$TEST_ROOT/tar_root"
-echo bar > "$TEST_ROOT/tar_root/foo"
-tar cvf "$TEST_ROOT/tar.tar" -C "$TEST_ROOT/tar_root" .
-path="$(nix flake prefetch --refresh --json "tarball+file://$TEST_ROOT/tar.tar" | jq -r .storePath)"
-[[ $(cat "$path") = bar ]]
-
-# Test a tarball that has a single executable regular file.
+# Test a tarball that has a single regular file.
 rm -rf "$TEST_ROOT/tar_root"
 mkdir -p "$TEST_ROOT/tar_root"
 echo bar > "$TEST_ROOT/tar_root/foo"