reproduce panic when parsing refspecs (#1508)

Author: Byron

gix/tests/fixtures/generated-archives/make_rev_spec_parse_repos.tar

@@ -417,3 +417,11 @@ git init new
 (cd new
   baseline '@{1}'
 )
+
+git init invalid-head
+(cd invalid-head
+  >file && git add file && git commit -m "init"
+  rm .git/refs/heads/main
+  baseline 'HEAD'
+  baseline 'HEAD:file'
+)

gix/tests/fixtures/make_rev_spec_parse_repos.sh

@@ -144,6 +144,16 @@ fn access_blob_through_tree() {
     );
 }
 
+#[test]
+fn invalid_head() {
+    let repo = repo("invalid-head").unwrap();
+    let err = parse_spec("HEAD", &repo).unwrap_err();
+    assert_eq!(err.to_string(), "The rev-spec is malformed and misses a ref name");
+
+    let err = parse_spec("HEAD:file", &repo).unwrap_err();
+    assert_eq!(err.to_string(), "TBD");
+}
+
 #[test]
 fn empty_tree_as_full_name() {
     let repo = repo("complex_graph").unwrap();